Diagram of protein 4.1R isoforms lacking AUG-1 and protein 4.1R isoforms lacking AUG-1 and AUG-2. 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. (A) The seven distinct protein 4.1R isoforms produced by initiation of translation at AUG-2 are shown, along with the percentage that each splice variant represented of the total clones analyzed. (B) The two 4.1R isoforms that lacked both exon 2′ and exon 4, the exons containing, respectively, AUG-1 and AUG-2.