Figure 4. Exon organization of xlProminin-1, 2, and 3. Homologous exons of xlProminin-1, 2, and 3 are aligned. Genes of all three xlProminins are alternatively spliced and their exon organization is evolutionarily conserved. Constitutive exons are marked in black. Alternatively spliced exons are marked orange. Spliced forms identified in cDNA clones are indicated by joining lines. Translation start sites (ATG) are marked with green lines and the positions of translational stop codons are marked with white lines. Note the complex splicing of exons 26b, 27, and 28a, which generates several distinct isoforms of xlProminin-1. Positions of primers used in reverse-transcription (RT)-PCR for semiquantitative analysis of mRNA expression and for alternative splicing are indicated on the diagram.