D-myo-Inositol-1,3,4,5-tetraphosphate (Ins(1,3,4,5)-P4) is formed by the phosphorylation of Ins(1,4,5)P3 by inositol 1,4,5-triphosphate 3-kinase.{783,1769} Ins(1,3,4,5)-P4 increases intracellular calcium levels by two distinct mechanisms: opening calcium channels on both the endoplasmic reticulum to release calcium from internal stores and on the plasma membrane to allow the influx of calcium from outside the cell.{831}