Classification of alternative and constitutive nucleotides. The procedure to classify nucleotides as alternative or constitutive is outlined in steps 1–4. Start and stop codons are marked by arrows and asterisks, respectively. Alternative regions of 5'UTR and CDS are colored dark blue and dark red, respectively, whereas constitutive regions are colored light blue and light red. Variable regions (those classified as alternative or constitutive, depending on the isoform) are colored gray. Alternative nucleotide positions are labeled "A" along the genomic sequence, while constitutive nucleotide positions are labeled "C".