Ins(1,2,3,5)P4 is one of the many inositol phosphate isomers that could act as small, soluble second messengers in the transmission of cellular signals. Ins(1,2,3,5)P4 exhibits relatively weak agonist activity at the Ins(1,4,5)P3 receptor expressed in CHO cells, where it induces Ca2+ mobilization with an ED50 vlau of 93.3 µM, making it nearly 1,000-fold less effective than Ins(1,4,5)P3 in the same assay.