To investigate whether azathioprine-associated mutagenesis contributes to the high incidence of skin tumours in organ transplant recipients (OTRs), we analysed PTCH gene mutations in 60 basal cell carcinomas (BCC); 39 from OTRs receiving azathioprine and 21 from individuals never exposed to azathioprine. This evidence concerns the gene PTCH1 and skin basal cell carcinoma.