Looking at the bases that precede the mutated base in C > T transitions (T for APOBEC3A, APOBEC3B, APOBEC3C and APOBEC1 or C for APOBEC3G) and considering the role of alkylating agents on G > A transitions, deaminating enzymes of the APOBEC family and alkylating agents are suspected to be the major causes of G:C > A:T mutations in Rwandan patients with gastric carcinoma. Here, APOBEC3B is linked to gastric carcinoma.