Diagram of protein 4.1R isoforms produced by initiation of translation at AUG-1. At the top is pictured 4.1R mRNA depicting the alternatively spliced exons, constitutive exons, and noncoding exons. The two translation initiation sites, AUG-1 and AUG-2, are in exon 2′ and exon 4, respectively. The nine distinct protein 4.1R isoforms produced by initiation of translation at AUG-1 are depicted below, along with the percentage that each splice variant represented of the total clones analyzed.