Figure 6. Alternative N-terminal kinases in the human genome. (A) Exon maps of kinases with alternative first exons that either do or do not contain palmitoylation sites, as occurs with BTK-A and BTK-C. (B) Alternate N terminal sequences of tyrosine kinases TXK, FRK, HCK, ABL1, ABL2, and the serine/threonine kinase PDPK1 resemble BTK. Isoforms containing palmitoylation sequences in the amino terminal region are enriched in cysteine and methionine residues in predicted protein products. The encoded alternative isoforms represent a binary switch as non-palmitoylated forms are depleted of cysteine and non-start codon methionine residues near the N terminus.