for the CAAT box, is located about 60–100 bases upstream (towards the 5' end), however no less than 27 base pairs away, from the initial transcription site or a eukaryote gene in which a complex of general transcription factors bind with RNA polymerase II prior to the initiation of transcription. It is essential to the transcription that these core binding factors (also referred to as nuclear factor Y or NF-Y) are able to bind to the CCAAT motif. Experiments in many laboratories have shown that mutations to the CCAAT motif that cause a loss of CBF binding also decreases transcriptional activity in these promoters, suggesting that CBF-CCAAT complexes are essential for optimum transcriptional activity. Binding In an experiment done with core binding factors (CBF) and DNA complexes, researchers were able to determine the preferential sequences of the promoter in a region over and immediately adjacent to the CAAT box, and two regions on either side of the CAAT box. By using PCR-mediated random binding selection process, researchers were able to show that the sequence "3' - (T/C)G ATTGG (T/C)(T/C)(A/G) - 5'" immediately flanking the ATTGG region (CCAAT in the complementary strand) was preferentially selected on the coding strand (opposite of the template strand). This was shown using an oligonucleotide sequence (R1) which contained 27 random nucleotides, flanked by a defined 20 nucleotide sequence on each side. While no single nucleotide was selected in every clone on either side of the ATTGG motif (CCAAT in the complementary strand), there were several nucleotides in positions selected with high frequency. Most notably from the sequence above was the G residue towards the 5' end of the ATTGG. The other residues also listed were notable, but there is a split between two residues. This same experiment also yielded the same sequence as shown above when using a different oligonucleotide (R2) that contained an ATTGG core and flanked by 12 5' random nucleotides and 10 3' random nucleotides. Both these sequences are very similar and confirmed in multiple experiments. For sequences that flanked the ATTGG motif with two adenine residues (AA) on its 5' end and G(A/G) on its 3' end, seems to have inhibited formation of the CBF-DNA complex and subsequently occurred in only 1% of the promoter sequences. In another experiment performed with the major late promoter (MLP) of adenoviruses from a variety of host species, it was shown that the mutation of