Familial Pseudohyperkalemia (FP) is a dominant red cell trait characterized by increased serum [K+] in whole blood stored at or below room temperature, without additional hematological abnormalities. Functional gene mapping and sequencing analysis of the candidate genes within the 2q35–q36 critical interval identified—in 20 affected individuals among three multigenerational FP families—two novel heterozygous missense mutations in theABCB6gene that cosegregated with disease phenotype. The two genomic substitutions altered two adjacent nucleotides within codon 375 of ABCB6, a porphyrin transporter that, in erythrocyte membranes, bears the Langereis blood group antigen system. The ABCB6 R375Q mutation did not alter the levels of mRNA or protein, or protein localization in mature erythrocytes or erythroid precursor cells, but it is predicted to modestly alter protein structure. ABCB6 mRNA and protein levels increase duringin vitroerythroid differentiation of CD34+ erythroid precursors and the erythroleukemia cell lines HEL and K562. These data suggest that the two missense mutations in residue 375 of the ABCB6 polypeptide found in affected individuals of families with chromosome 2-linked FP could contribute to the red cell K+ leak characteristic of this condition. Am. J. Hematol. 2013. © 2012 Wiley Periodicals, Inc.