c-Ki-ras mutations and gains of chromosome 12, where this gene is mapped, were both studied in a series of 47 colorectal cancers. Mutations at codon 12 and 13 were detected in 17 (36%) and gains of chromosome 12 in 7 (15%) cases. In this sample, gains of chromosome 12 occur in tumors either with or without c-Ki-ras mutations, suggesting that gains of chromosome 12 are independent from the mutation of c-Ki-ras.