Transcriptional repressor (By similarity). Regulates mesoderm versus endoderm differentiation during gastrulation to regulate kidney versus vascular cell fate. Acts upstream of pax2a in kidney development and functions in glomerular maturation, after podocyte specification. Expressed in the germ ring mesendoderm and subsequently in the endoderm and intermediate mesoderm, prior to the expression of definitive kidney or blood markers. At 60 hpf, expressed in the glomerulus, in some patches in the eye and brain, and weakly in the pectoral fin buds. Embryos show a down-regulation of early pronephric markers lhx1a/lim1 and pax2a/pax2.1, and subsequent kidney defects. Belongs to the Odd C2H2-type zinc-finger protein family. Contrary to PubMed:17011543, which shows pronephric expression during tailbud and somitogenesis stages, PubMed:18787069 report that osr1 is not expressed in kidney tissue. Similarly, in the absence of osr1, PubMed:17011543 report a complete absence of kidney tissue, and pericardial edemas characteristic of renal failure, whereas PubMed:18787069 report a segment-specific defect in the proximal nephron, with the distal nephron relatively unaffected, and a compensatory increase in angioblast cell numbers in the same trunk region.