*3.1. Phylogenetic Tree*

Phylogenetic analyses of human ABCA genes were performed by several previous studies. Some of them are based on the alignment of the nucleotide sequences or amino acid sequences of nucleotide-binding domains [3,20], the others on the alignment of the full length amino acid sequences [6,13]. Pairwise comparison of the amino acid sequences of all human ABCA members revealed homologies ranging from 28% (ABCA8/ABCA12) to 72% (ABCA8/ABCA9) [20]. The previous studies suggested that all ABCA genes have evolved from a primordial ancestor gene. Furthermore, they demonstrated that the human ABCA1, 2, 3, 4, 7, and 12 transporters cluster in a subgroup, distinct from ABCA5, 6, 8, 9, and 10. The ABCA5-related transporters share strikingly high overall amino acid sequence homologies but differ significantly from other members [3,6,13,20]. In contrast, our phylogenetic analysis, based on the comparison of nucleotide sequences of ABCA 5′UTRs, shows that the 5′UTRs of ABCA2 and ABCA3 cluster distinctly from the rest of the 5′UTRs. These discrepancies suggest that the evolution of 5′UTRs is shaped by different pressures independently from the other gene regions.
