Pleckstrin homology -PH) domains all contain a seven-standard .beta.-sandwich that allows binding to various species of phosphatidyl inositol -PtdIns) phosphates. Most PH domains bind to phosphatidylinositol phosphates -PIPs) with weak affinity and low specificity, however, a small subclass does bind specifically and with high-affinity for certain PIPs. PtdIns--4)-P1 Binding Protein contains a highly specific PH domain that recognizes and binds PtdIns--4)-P1. PIPs represent a small percentage of total membrane phospholipids however, they play a critical role in the generation and transmission of cellular signals. PtdIns--4)-P1 can phosphorylated by kinases to give bi- and triphosphates such as PtdIns--4,5)-P2 and PtdIns--3,4,5)-P3. Phosphorylation of PtdIns--4)-P1 by phosphoinositide -PI)3-kinase to make PtdIns--3,4)-P2 initiates an intricate signalling cascade that has been implicated in cancer.