BRCA1 and rectal cancer: On further comparison of right and left colon and rectal cancers, TP53 and APC were significantly more frequently mutated in rectal than right colon tumors, whereas the converse was true for PIK3CA, CTNNB1, ATM, PTEN, and BRCA1. Notably, BRAF, PIK3CA, CTNNB1, ATM, and PTEN showed decreasing mutational frequencies from the right side to the left side of the colon to the rectum, while increasing mutation frequencies of TP53 and APC were seen.