This report describes a rare polymorphism at codon 213 (silent alteration of CGA to CGG) within the coding region of the p53 gene. The rare polymorphic allele was present in six cases out of 189 lung and breast cancer DNAs analyzed (3.2%) and resulted in the loss of a TaqI site. This allele could be mistaken for a mutation when screening methods of mutation analysis are used without comparison with normal tissue DNA.