The Wiskott-Aldrich syndrome (WAS) is a primary X-linked immunodeficiency disease caused by mutations of the Wiskott-Aldrich syndrome protein (WASP) gene. The present molecular studies of six Japanese WAS patients identified five different mutations of WASP, including two novel mutations (45delG, 395insGGAGAT), the latter appearing to have occurred de novo. Familial carriers were detected by polymerase chain reaction-single strand conformational polymorphism analysis, restriction enzyme digestion and direct sequencing of PCR products. Neither mRNA nor the protein product were detectable in any of the patients, while various amounts of WASP protein were expressed in carriers, normal controls, haematopoietic cell lines of all lineages and in one patient after receiving allogeneic bone marrow transplantation.