F2 and colorectal cancer: Several small case-control studies have applied regression analyses to examine whether carriers of single thrombophilia gene polymorphisms, including Factor V Leiden (rs6025) and prothrombin G20210A (rs1799963) are at increased risk of cancer.[58] Two groups previously reported that prothrombin G20210A was associated with an increased risk of gastrointestinal and colorectal cancer respectively.[39,41] In contrast, Vossen et al[40] found that heterozygous carriers of either prothrombin G20210A or Factor V Leiden had a reduced risk of colorectal cancer.