Cellular SRC (c-SRC), which is the human homolog of the Rous sarcoma viral oncogene, v-src, is highly activated in colorectal cancer. Recently, a subset of colon cancers have shown a nonsense mutation at codon 531, which truncates c-SRC directly C-terminal to the c-SRC kinase regulatory domain. This specific mutation has been demonstrated to be activating, transforming, and tumorigenic and to promote metastasis. We investigated 100 rectal cancers, half of which had tumor spread outside the rectum, for this mutation by using direct sequencing. None of these tumors displayed any genetic alteration at this locus, and we thus conclude that the codon 531 mutation is a rare cause of c-SRC activation in rectal cancer.