To understand the role of p53 gene family members during invertebrate embryonic development, we used polymerase chain reaction (PCR) to identify p63/73 homologues in the marine mollusc Spisula solidissima. Here, we report the sequences of two distinct p63/73-like homologues, both cloned from Spisula embryos. The first, Ssp63/73alpha is 2699 nucleotide (nt); the second, Spp63/73beta is 3920 nt. The nucleotide sequences of the two variants are nearly identical up to their stop codons but diverge in their 3'-untranslated regions (UTRs). The deduced amino acid sequence of both Ssp63/73 variants is 597 amino acids, coding for a protein with predicted molecular weight of approximately 68 kDa. We conclude that the two unique transcripts, containing 3' UTRs of variable lengths, represent tandem alternate polyadenylation sites for the Ssp63/73 gene. While alternative splicing has been well documented in the p63/73 gene family, this is the first report of alternate polyadenylation site choice as a control point for p63/73 gene expression in any species. In order to identify specific post-transcriptional as well as post-translational signals potentially involved in regulation of p63/73-like expression, we compared Ssp63/p73 nucleotide and Ssp63/73 deduced amino acid sequences to corresponding regions of other mammalian and nonmammalian p63 and p73 homologues. Within the Spisula 3' UTRs we identified multiple AU-rich elements (AREs) which may control translation activation. Within the deduced amino acid sequence, we identified potential sites for sumoylation, a post-translational process that has been identified in mammalian p63 and p73 proteins. Identification of these novel signaling sites provides information about potential mechanisms controlling expression of multiple p63/73 isoforms during development.