Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit delta isoform also known as phosphoinositide 3-kinase (PI3K) delta isoform or p110δ is an enzyme that in humans is encoded by the PIK3CD gene. p110δ regulates immune function. In contrast to the other class IA PI3Ks p110α and p110β, p110δ is principally expressed in leukocytes (white blood cells). Genetic and pharmacological inactivation of p110δ has revealed that this enzyme is important for the function of T cells, B cell, mast cells and neutrophils. Hence, p110δ is a promising target for drugs that aim to prevent or treat inflammation, autoimmunity and transplant rejection. Phosphoinositide 3-kinases (PI3Ks) phosphorylate the 3-prime OH position of the inositol ring of inositol lipids. The class I PI3Ks display a broad phosphoinositide lipid substrate specificity and include p110α, p110β and p110γ. p110α and p110β interact with SH2/SH3-domain-containing p85 adaptor proteins and with GTP-bound Ras.