The p63 gene, a member of the p53 gene family, is expressed into at least six protein isoforms which are divided into two groups, those containing the transcription activation domain (TA isoforms) and those that do not (Delta N isoforms). The TA isoforms are similar to p53 in that they are able to activate transcription of specific target genes and induce cell cycle arrest and apoptosis. The Delta N isoforms are unable to activate transcription, and act in a dominant negative manner, inhibiting transcription activation by both p53 and TA isoforms. p63 knock-out studies in mice have shown that p63 plays an important role in development rather than in tumour suppression. In humans, mutations in the p63 gene have been linked with several developmental abnormalities. Studies on human tumours suggest an oncogenic function for Delta N isoforms rather than a tumour suppressor function for the TA forms.