The originally identified isoform+, with long signal peptide+ of 48 amino acids+ (IL-15 LSP) consisted of a 316 bp 5’-untranslated region (UTR), 486 bp coding sequence+ and the C-terminus 400 bp 3’-UTR region. The other isoform (IL-15 SSP) has a short signal peptide of 21 amino acids encoded by exons+ 4A and 5.