We have set up collaborations to test familial hemophagocytic lymphohistiocytosis (FHLH) patient samples from multiplex families in North America without known mutations, as well as inbred Pakistani families in which disease was previously mapped to chromosome 9q21. Sequencing of a novel candidate gene within the mapped locus is in progress for all samples. Antibodies have been generated against this protein to aid in its biochemical characterization. Functional studies have been initiated to ascertain how this protein may participate in granule-mediated cytotoxicity.