Inositol 1,4,5-trisphosphate (InsP(3)) 3-kinase catalyzes the phosphorylation of InsP(3) to inositol 1,3,4,5-tetrakisphosphate (InsP(4)). cDNAs encoding three isoenzymes of InsP(3) 3-kinase (3-kinases A, B, and C) have been previously reported; however, a demonstrably full-length cDNA encoding hum an InsP(3) 3-kinase B was still lacking. Here we report the cloning of a full-length 2841-bp cDNA encoding human InsP(3) 3-kinase B. Northern blot analysis shows the presence of an ubiquitous transcript of approximately 7.2 kb in a large number of human tissues. InsP(3) 3-kinase activity measured in COS-7 cells transfected with InsP(3) 3-kinase B shows an activity that was 8-fold increased upon the addition of Ca2+/calmodulin in the assay mixture. (C) 2002 Elsevier Science (USA).