The p53 tumor suppressor protein plays a critical role in the regulation of the cell cycle and apoptosis. The importance of p53's functions is underscored by the high incidence of p53 mutations in human cancers. Recently, two p53-related proteins, p73 and p63, were identified as members of the p53 gene family. Multiple isoforms of p73 have been found, including DeltaN variants in which the N-termini are truncated. p63 is expressed as three major forms, p63alpha, p63beta and p63gamma, each of which differ in their C-termini. All three forms can be alternatively transcribed from a cryptic promoter located within intron 3, producing DeltaNp63alpha, DeltaNp63beta and DeltaNp63gamma. The high degree of similarity of p73 and p63 to evolutionarily conserved regions of p53 suggests that these proteins play an important and potentially redundant role in regulating cell cycle arrest and apoptosis. Here we describe the characterization of cell lines generated to inducibly express p63alpha and DeltaNp63alpha. We have found that p63alpha and DeltaNp63alpha can differentially regulate endogenous p53 target genes and induce cell cycle arrest and apoptosis. Deletion of the N-terminal 26 amino acids of DeltaNp63alpha abolished its ability to transactivate p53 target genes and induce cell cycle arrest and apoptosis. This indicates that a putative transactivation domain exists within the N-terminus of the DeltaN variants of p63. Furthermore, the differential regulation of p53 target genes by p63alpha and DeltaNp63alpha suggests that p63 and p53 utilize both similar and different signaling pathways to execute their cellular functions.