Phosphatidylinositol 3-kinase is a lipid kinase that phosphorylates the inositol ring of phosphatidylinositol and related compounds at the 3’ position. PI 3-kinase p55 (PIK3R3) is comprised of a catalytic subunit and a regulatory subunit. The human p55 protein is composed of a rare amino terminal region followed by a proline-rich motif and two Src homology 2 (SH2) domains. PI 3-kinase p55 mRNAs are expressed in most human fetal and adult tissues; predominant expression is observed in the adult testis. Splice variant(s) of PI 3-kinase p55 have been identified; one of which has a deletion of 36 amino acids at the amino terminus and another which has an insertion of 59 amino acids at position 256 between the SH2 domains. Research suggests that PI 3-kinase p55 interacts with the IGFIR (Insulin-like growth factor-I receptor) and IR (Insulin receptor) and may be involved in PI 3-kinase activation by these receptors.