Many human cancers contain a hemizygous point missense mutation in p53, allowing expression of both wild-type and mutant p53. To understand the relationship between wild-type and mutant p53 in cells, we investigated the influence of a naturally occurring temperature-sensitive mutant p53 (valine to alanine substitution at codon 143: mp53-143ala) on the life span of normal human oral keratinocytes (NHOK) and the expression of wild-type p53. We also investigated the effect of the mutant p53 on the genetic stability of NHOK. The mp53-143ala extended the in vitro life span of NHOK by four-fold, but failed to overcome the M2 crisis stage for immortalization. The mp53-143ala notably suppressed wild-type p53 in NHOK at post-transcriptional levels. Moreover, the mp53-143ala notably increased both spontaneous and genotoxic agent-induced mutation frequency of a shuttle vector in NHOK. These data indicate that mutant p53 induces genetic instability by, in part, inhibiting the expression of wild-type p53 through a dominant negative role in cells expressing both mutant and wild-type p53.