*3.6. 5*′*UTR Introns*

Four ABCA genes (33%) have no 5′UTR intron; the others (67%) have at least one. The median as well as mode number of 5′UTR introns is 1, maximum is 3 (*ABCA3* and *ABCA10*). The median length of 5′UTR introns in ABCA subfamily is 5746 nts, minimum 890 and maximum 24,163 (*ABCA1*). Approximately 35% of human genes have been indicated to harbor introns within 5′UTRs, with median intron size 2643 pb [22,36,37]. A strong barrier against the presence of more than 1 intron in 5′UTRs has also been suggested. In some cases, introns in 5′UTRs were reported to enhance gene expression [38]. Although Cenik et al. [36] found no correlation in 5′UTR intron presence or length with variance in expression across tissues, they observed an uneven distribution of 5′UTR introns amongst genes in specific functional categories. Contrary to the study of Cenik et al. [36], Lim et al. [39] described a

strong negative correlation between the number of 5′UTR exon-exon junctions (the junctions between exons after intron removal) and main ORF translation efficiency in the five multicellular eukaryotic species studied (human, mouse, zebrafish, fruit fly and thale cress). We demonstrated an above-average occurrence and length of 5′UTR introns in ABCA genes.
