A transplacental carcinogenicity study was conducted by exposing pregnant Swiss (CD-1) mice to 0, 50, 100, 200, or 300 mg 3'-azido-3'-deoxythymidine (AZT)/kg body weight (BW) daily for the duration of gestation (18-19 days) [National Toxicology Program,2006]. The incidence of alveolar/bronchiolar adenomas and carcinomas in the 200 and 300 mg/kg groups was significantly higher (P = 0.027 and 0.007, respectively) in male offspring, but not in females (P = 0.338 and 0.315, respectively). The purpose of the present study was to evaluate K-ras mutation status in lung tumors from the female offspring in AZT exposed groups and to determine whether at the molecular level there were signature K-ras mutations in lung tumors that were different from spontaneous tumors. K-ras mutation was detected by cycle sequencing of polymerase chain reaction (PCR)-amplified DNA, isolated from formalin-fixed, paraffin-embedded lung tumors. K-ras mutations were detected in 17 of 28 (61%) lung tumors from the female offspring in AZT exposed groups. No K-ras mutations were detected in the 8 tumors examined from the female control group. The predominant mutations were Codon 12 G-->T transversions in the 50, 100, and 300 mg/kg groups, and Codon 12 G-->C transversions in the 200 and 300 mg/kg groups. K-ras Codon 12 G-->T transversions (TGT mutations) may be induced by oxidative DNA damage and 8-oxoguanine (8-oxoG), while K-ras Codon 12 G-->C transversions (CGT mutations) may be due to further oxidative lesions of guanine and 8-oxoG.