Very recently, a single nucleotide polymorphism (SNP) in the PTPN22 gene encoding the Lyp (lymphoid-specific phosphatase) PTP has been shown to be associated with susceptibility to rheumatoid arthritis (RA) and Type 1 diabetes (T1D)4,5. These data are consistent with the known role for Lyp in suppressing T cell activation and with data showing T cell activation (and potentially autoreactivity) to be increased by the RA and T1D-associated PTPN22 variant. The PTPN22 risk allele has also been recently implicated in Grave’s disease and as such appears to represent a susceptibility allele for many autoimmune diseases. As a number of these conditions (RA, T1D etc.) frequently occur within members of PBC families, this PTPN22 variant is very likely to also be involved in PBC, a possibility we will directly investigate in this pilot study. The aim of this study is to analyze the frequency of a specific PTPN22 polymorphism in patients with PBC.