May act as an angiogenesis inhibitor. Expressed at the 4-cell stage embryo. Expressed at gastrula stage in the ventral blastopore lip and the anterior end of the notochord. Expressed at early neurula stage throughout the notochord, the posterior paraxial mesoderm, cranial neural crest, anterior edge of the neural plate and at the prospective midbrain-hindbrain boundary (MHB) or isthmus organizer. Expressed at late neurula stage in the posterior neural fold. Expressed at early tailbud stage at the MHB until tadpole stage. Expressed at tailbud stage in the branchial arches, ear placode and ventral mesoderm. The C-terminal AMOP domain plays an important role in the anti-angiogenic function of ISM1. Belongs to the isthmin family.