A congenital portosystemic shunt (PSS) is an abnormal blood vessel that allows portal blood to bypass the liver. When blood is diverted around the liver, toxins from the intestinal tract are not filtered by the liver and reach high concentrations in the systemic circulation resulting in clinical signs, such as abnormal behavior and stunted growth. Many breeds of dogs are affected, but Yorkshire Terriers and Cairn Terriers are over-represented in clinical reports, and an inherited basis is suspected. A related condition called hepatoportal microvascular dysplasia (HMD) has been described in Cairn Terriers, Yorkshire Terriers, and several other breeds. The goals of this proposal are to carry out the clinical characterization of PSS and HMD in Cairn Terriers and Yorkshire Terriers, to collect pedigrees for pedigree analysis for the mode of inheritance, and to save DNA samples from this group of dogs so that molecular studies can be performed to further our understanding of the relationship between HMD and PSS and the mode of inheritance. Further linkage studies can be to identify DNA marker(s) which would help to identify carriers of this genetic defect so that breeders can avoid these problems in the future.