Mutations of MYH9, the gene for non-muscle myosin heavy chain IIA (NMMHC-IIA), cause a complex clinical phenotype characterized by macrothrombocytopenia and granulocyte inclusion bodies, often associated with deafness, cataracts and/or glomerulonephritis. The pathogenetic mechanisms of these defects are either completely unknown or controversial. In particular, it is a matter of debate whether haploinsufficiency or a dominant-negative effect of mutant allele is responsible for hematological abnormalities. We investigated 11 patients from six pedigrees with different MYH9 mutations. We evaluated NMMHC-IIA levels in platelets and granulocytes isolated from peripheral blood and in megakaryocytes (Mks) cultured from circulating progenitors. NMMHC-IIA distribution in Mks and granulocytes was also assessed. We demonstrated that all the investigated patients had a 50% reduction of NMMHC-IIA expression in platelets and that a similar defect was present also in Mks. In subjects with R1933X and E1945X mutations, the whole NMMHC-IIA of platelets and Mks was wild-type. No NMMHC-IIA inclusions were observed at any time of Mk maturation. In granulocytes, the extent of NMMHC-IIA reduction in patients with respect to control cells was significantly greater than that measured in platelets and Mks, and we found that wild-type protein was sequestered within most of the NMMHC-IIA inclusions. Altogether these results indicate that haploinsufficiency of NMMHC-IIA in megakaryocytic lineage is the mechanism of macrothrombocytopenia consequent to MYH9 mutations, whereas in granulocytes a dominant-negative effect of mutant allele is involved in the formation of inclusion bodies. The finding that the same mutations act through different mechanisms in different cells is surprising and requires further investigation.