Inositol-tetrakisphosphate 1-kinase (ITPK1) can phosphorylate inositol polyphosphates Ins(3,4,5,6)P4 at position 1 to form Ins(1,3,4,5,6)P5. This reaction is thought to have regulatory importance, since Ins(3,4,5,6)P4 is an inhibitor of plasma membrane Ca2+-activated Cl- channels.