Phosphoinositide 3-kinases (PI3-kinases) have been shown to be recruited to cell surface receptor signal complexes whose form ation is triggered by growth factors, cytokines and other ligands. P13-kinases are also involved in protein sorting phenom ena. A num ber of P13-kinase isotypes have been characterised in several laboratories. H ere the relations between the P13-kinases, P14-kinases and P15-kinases and other potential phosphoinositide kinases are analysed. A study of the relation of structure to function for sequence motifs defined through the use of homology searches and protein m odelling techniques is described and used to assign the family of phosphoinositide kinases to subgroups.