Intra-tumour heterogeneity for p53 status has been demonstrated in a number of different tumour types, and the identical nature of the mutation identified in both SJSA-1 derived lines, and of one of the three p53 mutations found in NGP derived lines demonstrated here, suggests that these were present in the original populations at low frequencies and were selected for during the course of treatment. This evidence concerns the gene TP53 and neoplasm.