There is evidence for the existence of a long XLαs transcript, termed XXLαs, which extends in the 5’ direction (Fig. 3A). The XXLαs transcript is supported by Est databases and a single study that could amplify portions of the XXLαs transcript by RT-PCR [72]. A long XLαs transcript has also been revealed by Northern blot in mouse heart and adipose tissue [67]. In fact, based on existing data, the translation initiation codon for XLαs does not match with that corresponding to the longest open reading frame (ORF), making it likely that an N-terminally extended XLαs variant is expressed at least in some tissues. Est databases also suggest the existence of transcripts that correspond to XXLαs-N1, but neither the role of XXLαs nor the role of XXLαs-N1 is currently known. Interestingly, however, unlike the XL domain in XLαs and XLαs-N1-- the portion encoded by exon XL -- the N-terminally extended part in XXLαs and XXLαs-N1 is highly conserved among mammals, suggesting that it may represent a functionally significant domain.
