Mutations in ROR2 result in a spectrum of genetic disorders in humans that are classified, depending on the nature of the mutation and the clinical phenotype, as either autosomal dominant brachydactyly type B (BDB,MIM 113000) or recessive Robinow syndrome (RRS, MIM 268310). In an attempt to model BDB in mice, the mutation W749X was engineered into the mouse Ror2 gene. In contrast to the human situation, mice heterozygous for Ror2W749FLAG are normal and do not develop brachydactyly,whereas homozygous mice exhibit features resembling RRS. Furthermore, both Ror2W749FLAG/W749FLAG and a previously engineered mutant, Ror2TMlacZ/TMlacZ, lack the P2/P3 joint. Absence of Gdf5 expression at the corresponding interzone suggests that the defect is in specification of the joint. As this phenotype is absent in mice lacking the entire Ror2 gene, it appears that specification of the P2/P3 joint is affected by ROR2 activity. Finally, Ror2W749FLAG/W749FLAG mice survive to adulthood and exhibit phenotypes (altered body composition, reduced male fertility) not observed in Ror2 knockout mice, presumably due to the perinatal lethality of the latter. Therefore, Ror2W749FLAG/W749FLAGmice represent a postnatal model for RRS, provide insight into the mechanism of joint specification, and uncover novel roles of Ror2 in the mouse.