Class IA phosphoinositide 3'-kinases (PI3Ks) regulate many cellular processes downstream of tyrosine kinases and Ras. Despite a clear implication of PI3K in cancer, little is known about the distribution of the different PI3K isoforms in malignant cells. We screened a large panel of tissues and cell lines for expression of class IA PI3Ks, and document a ubiquitous expression of the p110alpha and p110beta isoforms but a variable and more restricted tissue distribution of the p110delta isoform. Originally found in WBCs, p110delta was also detected in some nonhematopoietic cell types especially those of breast or melanocytic origin, both in the untransformed and transformed state. Isoform-specific neutralization of PI3K isoforms in breast cancer cell lines (by PI3K antibody microinjection or a p110delta-selective pharmacological inhibitor) demonstrated that p110delta is the most important class IA PI3K in the regulation of epidermal growth factor-driven motility in vitro, controlling the directionality and, to a lesser extent, the speed of migration. In contrast, p110beta was required for the direction but not the speed of migration, whereas p110alpha did not impact on either of these parameters. These results show a nonredundant function of PI3K isoforms downstream of the epidermal growth factor receptor and indicate that the presence of p110delta may confer breast cancer cells with selective migratory capacities. The potential clinical implications of p110delta expression in non-WBC-derived tumors are discussed.