AbstractCD157/Bst1 is a dual-function receptor and β-NAD⁺-metabolizing ectoenzyme of the ADP-ribosyl cyclase family. Expressed in human peripheral blood neutrophils and monocytes, CD157 interacts with extracellular matrix components and regulates leukocyte diapedesis via integrin-mediated signalling in inflammation. CD157 also regulates cell migration and is a marker of adverse prognosis in epithelial ovarian cancer and pleural mesothelioma. One form of CD157 is known to date: the canonical sequence of 318 aa from a 9-exon transcript encoded by BST1 on human chromosome 4. Here we describe a second BST1 transcript, consisting of 10 exons, in human neutrophils. This transcript includes an unreported exon, exon 1b, located between exons 1 and 2 of BST1. Inclusion of exon 1b in frame yields CD157-002, a novel proteoform of 333 aa: exclusion of exon 1b by alternative splicing generates canonical CD157, the dominant proteoform in neutrophils and other tissues analysed here. In comparative functional analyses, both proteoforms were indistinguishable in cell surface localization, specific mAb binding, and behaviour in cell adhesion and migration. However, NAD glycohydrolase activity was detected in canonical CD157 alone. Comparative phylogenetics indicate that exon 1b is a genomic innovation acquired during primate evolution, pointing to the importance of alternative splicing for CD157 function.