AbstractWe have studied 31 male germ cell tumors (GCTs) for probable mutations in codons 12, 13, and 61 ofHRAS, KRAS, andNRASoncogenes using the polymerase chain reaction. Twenty of the thirty‐one tumors exhibitedNRASgene mutations, 14 in codon 61, and six in codon 12, whereas no mutations were detected inHRASandKRASgenes. TheNRASmutations were equally prevalent in seminomatous and nonseminomatous GCTs. Thus 13 of 22 seminomas, six of seven embryonal carcinomas, and one of two mixed tumors exhibited mutations. Two non‐seminomatous tumors (an embryonal carcinoma and a yolk sac/teratoma) had mutations in both codons 12 and 61. The high frequency ofNRASmutations observed in the present study suggests that NRAS gene products may play an important role in growth regulatory functions of premalignant and malignant germ ce