This CCDS representation uses a downstream AUG start codon. This start codon has a strong Kozak signal and is restricted to human. An upstream start codon, which has a weak Kozak signal and is conserved in some other primates, is also present. The use of the upstream start codon would result in a protein that is 40 aa longer at the N-terminus. However, this would eliminate a signal peptide that is predicted for the shorter but not the longer N-terminus. PMID:9242611 shows experimental evidence for cleavage of a signal peptide following aa 25 from the downstream start, and this is also confirmed in PMID:15340161. Therefore, the downstream start codon is represented for this CCDS.