Based upon current functional analysis, the characterization of TACC3 orthologues is likely to be pivotal to understanding the sequence and functional evolution of the TACC gene family. As indicated below, the chromosomal region containing the TACC gene precursors was duplicated twice during vertebrate evolution. Although the analysis of T. rubripes, rodents and humans so far suggests that the vertebrate TACC3 precursor was not included in the second round of genomic duplication, it could not be excluded that a TACC4 gene may have been lost during the evolution of these lineages. The cloning of a new member of the TACC family in Oryctolagus cuniculus has added to this controversy [9]. Designated TACC4, the 1.5 kb cDNA was highly related, but proposed to be distinct from TACC3. However, Northern blot data suggested that this gene produces a single 2.3 kb transcript [9], indicating that the cloned cDNA was incomplete. The degree of similarity to the published sequence of human and mouse TACC3 suggested to us that TACC4 actually represents a partial rabbit TACC3 cDNA. To test this hypothesis, we set out to clone the complete rabbit TACC3 sequence, based upon the known features of human and mouse TACC3. We have previously noted that the N-terminal and C-terminal regions of the human and mouse TACC3 proteins are highly conserved ([2], see below). Therefore, based upon the sequence identity between these genes, we designed a consensus oligonucleotide primer, T3con2, that would be suitable for the identification of the region containing the initiator methionine of the TACC3 cDNAs from primates and rodents. Using this primer, in combination with the TACC4-specific RACE primer (RACE2), initially used by Steadman et al [9], we isolated a 1.5 kb PCR product from rabbit brain cDNA by rt-PCR. In combination with 3'RACE, this generated a consensus cDNA of 2283 bp which corresponds to the transcript size of 2.3 kb detected by the "TACC4" sequence reported in Figure 4 of Steadman et al [9]. Thus, while it remains possible that the "TACC4" sequence is an alternative splice product, or is the product of reduplication of the TACC3 gene (events that would be specific to the rabbit), the only transcript detected in rabbit RNA corresponds to the predicted transcript size of the TACC3 sequence that we have identified here. Furthermore, the string of nucleotides found at the 5' end of the "TACC4" sequence is also found at the 5' ends of a number of cDNA sequences (e.g. U82468, NM_023500), that were isolated by 5'RACE, suggesting that they may correspond to an artefact of the 5'RACE methodology used in their construction. The rabbit "TACC4" and the rabbit TACC3 sequence that we have isolated are also found on the same branch of the TACC phylogenetic tree with the other TACC3 orthologues, including maskin (Xenopus laevis), and the newly identified TACC3 sequences in Rattus norvegus, Gallus gallus, Silurana tropicalis, Danio rerio and T. rubripes, reported in this manuscript (Fig. 1). Thus, it is not in a separate branch that may be expected if the sequence was a distinct TACC family member.