Ras gene point mutation is a rare event in premalignant tissues and malignant cells and tissues from oral mucosal lesions.

Three series of biopsy specimens of premalignant and malignant oral lesions, together with seven human keratinocyte cultures, previously established from oral squamous cell carcinomas, were analysed for point mutation in exons 1 and 2 of the c-Ha-ras, c-Ki-ras and N-ras genes by direct nucleotide sequencing of DNAs amplified in the polymerase chain reaction (PCR). Only one out of 12 biopsy samples (8.3%), a well-differentiated carcinoma which was the latest in a series of floor of mouth lesions from 1 of the 3 patients studied, harboured a mutant c-Ha-ras gene, being heterozygous at codon 12 for a GGA-GTA change. One cell line (H357) showed heterozygosity in both exons 1 and 2 of c-Ha-ras, harbouring a GGT to AGT mutation over codon 13 and a CAG to CAA mutation over codon 61. The remaining six oral carcinoma cell lines (85.7%) were homozygous normal at both exons 1 and 2 of c-Ha-ras. All cell lines showed normal c-Ki-ras and N-ras loci. We conclude that ras gene mutation is an infrequent occurrence in the malignant progression of oral epithelial cells, despite the probable importance of chemical carcinogens in the aetiology of the disease. We emphasise the need to search for other cellular sequences which may be targets for chemical or viral carcinogens.