*3.2. 5*′*UTR Length*

The average/median length of 5′UTRs is 291/212 nts in the human ABCA genes; however, there is a great variation among the 12 members. The smallest 5′UTR is annotated to *ABCA13* (26 nts) and largest to *ABCA10* (910). In the literature, we find divergent data about the average/median length of human gene 5′UTRs based on the input data and year of study. Pesole and coworkers [21] constructed the first database focusing on the UTR sequences from different eukaryotic taxa and named it UTRdb. The average length of mRNA 5′UTR in humans was calculated to be 210 nts, maximum length 2803 and minimum 18 [22]. Rogozin et al. [23] calculated the average length of human 5′UTRs to be 160 nts (retrieved from EMBL database). In the work of Chen et al. [24], the average/median length of 5′UTRs in humans was calculated to be 254/169 nts from the Ensembl database and 220/160 from the UTRdb database. Recently, Leppek et al. [17] stated that the longest known median length of mRNA 5′ UTRs occurs in humans and is 218 nts (based on RefSeq data). We can conclude that the median length of 5 ′UTRs in the human ABCA subfamily is close to the median length derived from the whole genome data. Indeed, the genome average of 5′UTR lengths is relatively similar across diverse taxonomic classes of eukaryotes, ranging approximately from 100 to 200 nts, while in sharp contrast, the 5′UTR length varies considerably among the genes in a genome, from a few to several thousands of nts, a fact which has been mentioned in several previous studies [17,22,25,26]. Lynch and colleagues [27] suggested that this discrepancy can be explained by random genetic drift and mutational processes that cause stochastic turnover in transcription-initiation sites and premature start codons. Under the simple null model that they presented, natural selection only indirectly influences the lengths of 5′UTRs through the mutational origin of premature initiation codons within the UTR. We further confirmed in this study that the great length variability of 5′UTRs can be observed even in closely related members of a gene subfamily.
