The human isoforms of VEGF-A. At least five different isoforms are generated by alternative splicing from a single gene. Exon I, encoding for a signal peptide, exons IIV and exon VIII are present in all isoforms. Exons III and IV are required for binding to VEGFR-1 and VEGFR-2, respectively. The isoforms differ by the presence or absence of exon VI, that mainly mediates heparin binding and exon VII, which seems to be responsible for interaction with neuropilin-1. (Adapted from refs. ^5,31).