We report here a cancer drug therapy use of a gene involved in Down's syndrome. Using bioinformatics approaches, we recently predicted Single Minded 2 gene (SIM2) from Down's syndrome critical region to be specific to certain solid tumors. Involvement of SIM2 in solid tumors has not previously been reported. Intrigued by a possible association between a Down's syndrome gene and solid tumors, we monitored SIM2 expression in solid tumors. Isoform-specific expression of SIM2 short-form (SIM2-s) was seen selectively in colon, prostate, and pancreatic carcinomas but not in breast, lung, or ovarian carcinomas nor in most normal tissues. In colon tumors, SIM2-s expression was seen in early stages. Antisense inhibition of SIM2-s expression in a colon cancer cell line caused inhibition of gene expression, growth inhibition, and apoptosis. The administration of the antisense, but not the control, oligonucleotides caused a pronounced inhibition of tumor growth in nude mice with no major toxicity. Our findings provide a strong rationale for the genes-to-drugs paradigm, establish SIM2-s as a molecular target for cancer therapeutics, and may further understanding of the cancer risk of Down's syndrome patients.