We have isolated cDNA clones encoding an inositol 1,4,5-trisphosphate receptor type 1 (InsP3R1) from human uteri and a leukaemic cell line, HL-60. Northern-blot analysis showed that approx. 10 kb of InsP3R1 mRNA is expressed in human uteri, oviducts and HL-60 cells. The predicted amino acid sequence of human InsP3R1 (2695 amino acids) has 99% identity with that of the mouse SI-/SII- splicing counterpart. Western-blot analysis with anti-(mouse InsP3R1) antibodies showed that InsP3R1 protein of human uteri and oviducts of approx 220 kDa is immunostained. Northern-blot analysis of HL-60 cell differentiation along the neutrophilic lineage induced by retinoic acid or dimethylsulphoxide showed an accompanying enhanced expression of InsP3R1 mRNA. Immunohistochemical analysis of the cerebella of spinocerebellar degeneration patients showed a variable loss of Purkinje cells with an altered pattern of immunostaining. The InsP3R1 gene (Insp3r1) was localized to the 3P25-26 region of human chromosome 3. The data presented here clearly show that InsP3R1 exists widely in human tissues and may play critical roles in various kinds of cellular functions.