Our effort to identify novel drug-resistant genes in cyclophosphamide-resistant EMT6 mouse mammary tumors led us to the identification of SPF45. Simultaneously, other groups identified SPF45 as a component of the spliceosome that is involved in alternative splicing. We isolated the human homologue and examined the normal human tissue expression, tumor expression, and the phenotype caused by overexpression of human SPF45. Our analyses revealed that SPF45 is expressed in many, but not all, normal tissues tested with predominant expression in normal ductal epithelial cells of the breast, liver, pancreas, and prostate. Our analyses using tissue microarrays and sausages of tumors indicated that SPF45 is highly expressed in numerous carcinomas including bladder, breast, colon, lung, ovarian, pancreatic, and prostate. Interestingly, this study revealed that overexpression of SPF45 in HeLa, a cervical carcinoma cell line, resulted in drug resistance to doxorubicin and vincristine, two chemotherapeutic drugs commonly used in cancer. To our knowledge, this is the first study showing tumor overexpression of an alternate splicing factor resulting in drug resistance. The development of multidrug resistance is a major hurdle in the treatment of cancer. Although some genes that confer multidrug resistance are known, it is clear that many other genes remain to be identified. The characterization of novel mechanisms that lead to drug resistance would enable the development of a targeted new generation of anti-cancer drugs. To identify novel genes involved in drug resistance we performed suppressivesubtractive polymerase chain reaction analyses of the cyclophosphamide-resistant EMT6 mouse mammary tumor cells and the parental EMT6 tumors. This led to the identification of a gene that was highly homologous to a DNA damage response protein, DRT111 of Arabidopsis (unpublished observations). 1 While this work was in progress, Neubauer and colleagues, 1 identified a novel protein from the spliceosome. An analysis of all proteins present in the spliceosomal complex led to the identification of 25 previously identified proteins and 20 novel proteins. One of the novel proteins thus identified was named SPF45 (splicing factor 45 kd). This protein was identical to the one that was identified by our suppressive-subtractive polymerase chain reaction analysis. Neubauer and colleagues, 1 further showed that SPF45 was a nuclear protein that co-localizes with U1A snRNP in nuclear speckles, a known reservoir of splicing factors. [2][3][4] Recently Rappsilber and colleagues, 5 have described the identification of 311 proteins in the human spliceosomal complex of which 96 were novel proteins. Again, in this study, SPF45 was identified as a component of the spliceosome.Lallena and colleagues, 6 demonstrated that SPF45 regulates the alternate splicing of the Drosophila sexlethal gene (sxl). SXL protein is expressed only in female flies, but not in males. 7 Exon3 of Sxl plays a very important role in determining the expression of SXL because its inclus...