The p53 family of transcription factors has been implicated in many vertebrate cancers. Altered p53 and p73 protein expression observed in leukemic cells of molluscs suggests that these transcription factors might be involved in invertebrate cancers as well. Here, we fully characterize the mRNA of four novel p53-like variants in the bivalve molluscs Mytilus trossulus (bay mussel) and Mytilus edulis (blue mussel). These species, widely used for environmental assessment, develop a hemic neoplasia (leukemia) that is frequently fatal. The correlation between expression of p53 and its close relative p73 and onset of molluscan leukemia was documented previously. We report the sequences of two distinct and novel p63/p73-like mRNAs, amplified by polymerase chain reaction (PCR) from both species. One of the p63/p73-like isoforms contains a 360 nt truncation in the 5' coding region. Based on this truncation and concomitant lack of a transactivation (TA) domain, we designate this variant as a DeltaNp63/p73-like isoform: the first to be reported in an invertebrate species. In mammalian species, DeltaNp73 potently inhibits the tumor-suppressive function of p73 and p53, and its overexpression serves as a robust marker for mammalian cancer. In addition, we report on the occurrence of alternate polyadenylation sites in the molluscan p63/p73: one proximal and one distal site, which differ by 1260 nt. We hypothesize that differential expression of various molluscan p63/p73-like isoforms, controlled in part by polyadenylation site choice variation, may help to interpret the apparently opposing roles of this gene in the development of cancer. Overall, this research further illustrates the utility of the molluscan model for studies involving the molecular mechanisms of oncogenesis in naturally occurring populations. The data presented here require a revisiting of hypotheses regarding evolution of the p53 gene family. Current hypotheses indicate that (1) the protostome gene family does not contain an intronic promoter for DeltaN expression and (2) p53 gene duplication did not occur in protostomes. Our characterization of DeltaN p63/73 in mussel suggests that molluscan p53 gene family members have acquired an intronic promoter or splicing mechanism, either by invention that predates the evolutionary split of deuterostomes from protostomes, or by parallel evolution. Our data also show that Mytilus p53, p63/p73, and DeltaNp63/p73 are identical in their core regions with variation limited to their C- and N-terminals, supporting the notion that alternative splicing, intronic promoter usage, and polyadenylation site choice may lead to expression of distinct isoforms originating from one common gene.