The expression of the p53 tumor suppressor gene in ten human cell lines (nine cancers and one normal) was studied using reverse transcription, polymerase chain reaction (PCR) and direct sequencing. Using P53U and P53D primers for amplifying a 371-base pair (bp) target fragment spanning exons 7-10 of p53 cDNA, normal-sized PCR products were amplified from 9 cell lines but not from the Hep3B hepatocellular carcinoma (HCC) cell line. An additional larger band (504 bp) was observed for the Molt-4 T-lymphoblastic leukemia cell line. Employing P531 and P53D primers which flank a 76-bp p53 cDNA fragment, 76 bp as well as 209 bp products were generated by PCR of Molt-4 cDNA. Direct sequencing of the 504 bp and 209 bp bands confirmed the presence of a 133 bp insertion between exons 9 and 10 in the aberrant transcript. This insertion was homologous to a 130-bp sequence within the wild-type p53 intron 9, except for 2 point mutations and 3 base insertions. Sequencing of P53U/P53D PCR products of Molt-4 genomic DNA revealed an 8 bp deletion just downstream to the 133 bp insertion, creating a novel donor splicing site within intron 9. This site, coupled with an inherent acceptor splicing site just upstream to the 133 bp insertion, suggests that the 133 bp stretch represents an alternative exon. The occurrence of a termination signal within this alternative transcript is predicted to culminate in a truncated p53 translational product. The sequences of the 371 bp PCR products of Molt-4, HT-1080, SiHa, CaSki, HeLa and MRC-5 cell lines corresponded with the wild-type p53 cDNA. G-->T transversions at the third base of codon 249 of p53 were detected in Mahlavu and PLC/PRF/5 HCC lines, while a TAC to CAC mutation at codon 234 was observed in an allele of the Raji Burkitt lymphoma line.