Essential for the initiation of pectoral fin bud formation. Potentially acts downstream of retinoic acid and Wnt signaling and is required for tbx5 expression in the lateral plate mesoderm of presumptive pectoral fin bud regions. Homodimer; disulfide-linked. Seems to also exist as monomers (By similarity). First detected in the notochord at 10 hours post fertilization (hpf). At 12 and 14 hpf (the 6- and 10-somite stages), expressed in the notochord, somites and tail. At 14 hpf, also expressed in the forebrain and the lateral plate mesoderm anterior to somite 1. At 18 and 24 hpf, predominantly expressed in the tail, presumptive pectoral fin bud regions and the alar plate of the hindbrain. Belongs to the FIBIN family.