Hereditary thrombocythaemia (HT) is an autosomal dominant disorder with clinical presentation and complications resembling sporadic essential thrombocythaemia (ET). Mutations in the thrombopoietin (TPO) gene causing overproduction of TPO and elevated TPO serum levels have been found previously in three families with HT. Here, we present evidence for genetic heterogeneity by demonstrating that HT in a Spanish and a US family is caused by genes other than TPO. Affected family members in both families had normal TPO serum levels. Genetic linkage analysis with TPO microsatellite markers excluded TPO as the disease gene in the Spanish HT family, and sequencing of the TPO gene revealed no mutations in the propositus of the US family. To test a role for MPL, the gene for the TPO receptor, we identified three single nucleotide polymorphisms (SNP) and a novel polymorphic CA microsatellite marker. By linkage analysis, we excluded MPL as the cause of HT in the Spanish family. Interestingly, mapping of the CA microsatellite marker to a region 40.5 kb upstream of MPL revealed the presence of sequences from the TIE gene, which encodes a tyrosine kinase receptor expressed on megakaryocytes and endothelial cells. Thus, MPL and TIE are in close physical proximity, and the CA microsatellite described here will be a useful genetic marker for both genes.