factors. In this study, researchers showed that, in HeLa cells which derived from cells of a person who had cervical cancer and have proven useful for experiments, the first group of spliceosomes on pre-mRNA come from these speckles. Researchers used microinjections of spliceosome-accepting and mutant adenovirus pre-mRNAs with differential splicing factor binding to make different groups and then followed the sites in which they were heavily present. Spliceosome-accepting pre-mRNAs were rapidly targeted into the speckles, but the targeting was found to be temperature-dependent. The polypyrimidine tract sequences in mRNA promote the construction of spliceosome groups and is required for targeting, but, by itself, was not sufficient. The downstream flanking sequences were particularly important for the targeting of the mutant pre-mRNAs in the speckles. In supportive experiments, the behavior of the speckles was followed after the microinjection of antisense deoxyoligoribonucleotides (complementary sequences of DNA and or RNA to a specific sequence) and, in this case, specific sequences of snRNAs. snRNAs are known for helping in the processing of pre-mRNA as well. Under these conditions, spliceosome groups formed on endogenous pre-mRNAs. Researchers concluded that the spliceosome groups on microinjected pre-mRNA form inside the speckles. Pre-mRNA targeting and buildup in the speckles is a result of the loading of splicing factors to the pre-mRNA, and the spliceosome groups gave rise to the speckled pattern observed. Related diseases Research has also led to greater knowledge about certain diseases related to changes within primary transcripts. One study involved estrogen receptors and differential splicing. The article entitled, "Alternative splicing of the human estrogen receptor alpha primary transcript: mechanisms of exon skipping" by Paola Ferro, Alessandra Forlani, Marco Muselli and Ulrich Pfeffer from the laboratory of Molecular Oncology at National Cancer Research Institute in Genoa, Italy, explains that 1785 nucleotides of the region in the DNA that codes for the estrogen receptor alpha (ER-alpha) are spread over a region that holds more than 300,000 nucleotides in the primary transcript. Splicing of this pre-mRNA frequently leads to variants or different kinds of the mRNA lacking one or more exons or regions necessary for coding proteins. These variants have been associated with breast cancer progression. In the life cycle of retroviruses, proviral DNA is incorporated in transcription of the DNA of the cell being infected. Since retroviruses need to change their pre-mRNA into DNA so that this DNA can be integrated within the DNA of the host it is