FXYD5(FXYD domain-containing ion transport regulator 5), also known as Dysadherin, is a protein that in humans is encoded by the FXYD5 gene. The gene encodes a member of a family of small membrane proteins that share a 35-amino acid signature sequence domain, beginning with the sequence PFXYD and containing 7 invariant and 6 highly conserved amino acids. The FXYD5 gene is mapped on 19q13.12. The deduced FXYD5 protein, the largest of the 7, contains 178 amino acids and has a signal peptide. Immunoreactivity for dysadherin, localized at cell-cell boundaries, was detected in 39 of 51 papillary carcinomas and in all 31 undifferentiated carcinomas but not in the follicular carcinomas or normal thyroid tissue controls. Dysadherin expression was significantly higher in undifferentiated carcinoma than in papillary carcinoma or follicular carcinoma and showed significant negative correlation with E-cadherin expression.