there can be little doubt that the ability to use homologous recombination in embryonic stem ( es ) cells as a method to generate mice with defined genomic modifications has made them the premier model system for the study of mammalian development and organogenesis . this has been further enhanced by recombinase - based methods that allow mutations to be induced in a tissue- or temporal - specific fashion . however , the task of generating mice with genome modifications remains time- and resource - intensive , and is particularly daunting for investigators without experience in this domain . this problem has been recently addressed by the initiation of large - scale targeted and gene - trap mutagenesis efforts , many of which have progressed to maturity , providing a vast resource for investigators wishing to use the mouse as a model system . the following summarizes the present status of several of these programs . for additional information , a thorough review of the history and methodologies of these and other programs has recently been published . in 2004 , a proposal was made to pursue a large - scale project to generate a library of es cells carrying mutations of all known or presumptive genes . this effort has made remarkable progress , and very large numbers of targeted es cells are now available to investigators at modest cost . the project is being undertaken by a consortium whose diverse responsibilities include the generation of targeting vectors , the selection and characterization of mutant es cells , the distribution of reagents , and the presentation of data . the largest components of this effort are represented in the komp ( knock - out mouse project ) and eucomm ( european conditional mouse mutagenesis ) programs . csd is a collaborative team from the children 's hospital oakland research institute ( chori ) , the wellcome trust sanger institute , and the university of california at davis school of veterinary medicine that has a goal of generating 5000 targeted es cells . the csd strategy uses a remarkable high - throughput method of recombineering developed at the sanger institute to generate targeting vectors that will generate conditional mutant alleles . this is paired with an automated es cell colony - picking technology and allele characterization by long - range polymerase chain reaction ( pcr ) . the other major component of komp is an effort by investigators at regeneron to generate 3500 deletion alleles . this employs a method of bacterial artificial chromosome ( bac)-mediated homologous recombination and replacement that they have developed and adapted for high throughput . successful targeting is assayed by using quantitative pcr to discriminate between wild - type and hemizygous cell lines for each mutant allele . the komp has been highly productive ; as of may 2010 they have generated a total of 8092 targeting vectors , mutated es cell lines for 5643 loci , and generated over 300 mutant mouse lines . eucomm is a parallel effort with a goal of generating 8000 targeted conditional mutant alleles . the project was developed at the wellcome trust sanger institute , the helmholtz zentrum mchen german research center , the medical research council ( mrc ) mammalian genetics unit , harwell , uk , the institut clinique de la souris , strasbourg , france , the consiglio nazionale delle ricerche , monterotondo , italy , the university of frankfurt , germany , the center for cardiovascular research at the charit , berlin , germany , the university of technology , dresden , germany , and the european molecular biology laboratory ( embl ) , monterotondo , italy . eucomm has produced 5980 vectors , 3473 mutated es cells , and over 400 mutant mouse lines . a third major project involving gene targeting by homologous recombination is being pursued as part of the norcomm ( north american conditional mouse mutagenesis ) project . the goal in this effort is up to 500 targeted loci , and the targeting protocol maximizes the utility of the modified locus for further manipulation ( e.g. , replacement with specific mutation [  knock - in ] , different reporters , recombinases , and so on ) . these three projects ( and the texas a&m institute for genomic medicine , see below ) are associated in the international knockout mouse consortium ( ikmc ) . the ikmc data coordination center provides unified access to information about vector design and status of es cell line and mutant mouse generation . a genome - view perspective of alleles that have targeted mutations can be obtained using the  ikmc genes ribbon on the university of california santa cruz genome browser   or the  ko alleles das ( distributed annotation system ) track on the ensembl genome browser . a crucial aspect of any large - scale biological resource is facilitating distribution ; this is done by the komp repository at the university of california at davis   and by the european mouse mutant cell repository ( eummcr ) for targeted es cells or from the european mutant mouse archive ( emma ) for mutant mouse strains . the komp repository also provides blastocyst injection under a fee - for - service proviso . note that all vectors can be obtained for investigators still inclined to  do - it - yourself gene - targeting . a unifying aspect of these projects is the use of c57bl/6-derived es cells for targeting . this has been an area of ongoing discussion , in which the efficiency of germ - line passage obtained using 129-derived lines was balanced against the desirability of having targeted mutations isogenic on a c57bl/6 background . interestingly , this requirement did not specify a substrain , and it has turned out that c57bl/6n has been more robust for es cell line development than c57bl/6j . the feeder - independent jm8 cell line , which is the primary line used for targeting by the eucomm and komp projects , is c57bl/6n - derived . these targeted libraries complement several large gene - trap resources , such as that generated as part of the baygenomics project , the german gene trap consortium , the toronto centre for modelling human disease , and other efforts . data regarding the insertion sites , targeting vectors , and transcript characterization for all publicly available gene - trap cell lines are now curated and accessible via a central database maintained by the international gene - trap consortium . a recent addition is a library originally generated at lexicon genetics and now distributed by the texas a&m institute for genomic medicine . this library contains over 350,000 cell lines representing more than 10,000 unique genes , and is notable for being in c57bl/6n - derived es cells . however , the cost of mice generated from this resource is substantially more than that for mice obtained from most public - domain programs . while we are potentially on the cusp of a new era in the utilization of genetically modified mice for developmental analysis , a number of unknowns exist . first and foremost is the robustness of the newly developed resource with respect to the generation of germline - competent mutant mice . many variables affect this , including genetic background , genomic stability , culture conditions , and technical expertise . both the sanger institute and regeneron have excellent success in generating germline - competent mice in their own facilities . for example , targeted jm8 clones successfully colonized the germline in over 65% of experiments . as multiple cell lines have been derived for each targeted locus , the likelihood of obtaining a germline - competent mouse for any specific gene of interest is high . another issue is the fact that the resource has been created in a c57bl/6n - derived cell line . although the j and n substrains are very closely related ( only 102 of 139,561 genotyped single - nucleotide polymorphisms are discordant ) , several phenotypic differences have been noted   and molecular differences , such as a deletion of the nnt gene in c57bl/6j , have been found . finally , there is a spectrum of opinions regarding how such resources can best be utilized . the european community and the wellcome trust sanger institute have developed a vigorous effort to support systematized and centralized phenotyping in a large - scale fashion . while there are clear efficiencies and opportunities for discovery inherent by phenotypic screening in an unbiased fashion , there is also a case to be made for facilitating analysis by individual investigators who are invested in specific gene families , pathways , or phenotypes . as an example , relatively modest but targeted enu ( n - ethyl - n - nitrosourea ) mutagenesis programs have been arguably as productive , with respect to mutation discovery and functional characterization , as much larger , broadly focused efforts [ 18 - 21 ] . in summary , an extraordinary resource of gene deletions , disruptions , and conditional mutant alleles is now available at relatively low cost to investigators worldwide . the substantial investment required to create these libraries is likely to yield substantial insight into the role of specific genes in mammalian biology . drb is a member of the scientific advisory boards for the komp and eucomm programs .
the utilization of homologous recombination in embryonic stem cells as a means to generate mice carrying pre - determined modifications of genomic sequences has revolutionized the study of developmental biology . 
 recognizing the potential efficiencies that can be obtained by high - throughput production at centralized technology centers , a number of large - scale efforts for generating mice with targeted mutations have been funded . 
 these programs are reaching fruition , and a variety of libraries of embryonic stem cells with defined mutations are now available .