Familial pseudohyperkalaemia (FP) is a symptomless, dominantly inherited red cell trait, which shows a 'passive leak' of K+ cations into the plasma upon storage of blood at room temperature (or below). There are no haematological abnormalities. The loss of K+ is due to a change in the temperature dependence of the leak. The Scottish case initially described, FP Edinburgh, maps to 16q23-qter. Here we studied a large kindred of Flemish descent with FP, termed FP Lille, which was phenotypically identical to the Edinburgh FP. In FP Lille, however, the responsible locus mapped to 2q35-36, with a Lod score of 8.46 for marker D2S1338. We infer that FP Edinburgh and FP Lille, although they are phenocopies of one another, stem from two distinct loci, FP1 (16q23-qter) and FP2 (2q35-36), respectively. This duality hints at the possibility that the protein mediating the leak might be a heterodimer. No mutation was found in three plausibly candidate genes: the KCNE4 gene, the TUBA1 gene and a predicted gene located in genomic contig NT_005403.
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