Phosphoinositide 3-kinase is a lipid and protein kinase composed of a 110-kDa catalytic subunit and an 85-kDa (p85) or 55-kDa (p55) regulatory subunit. In mammals, at least two genes encode catalytic subunits, and at least three genes encode regulatory subunits. Here we report the cloning and structural analysis of the mouse p85α gene. The translated portion of mouse p85α is encoded by 15 exons that span at least 40 kb. We have cloned an alternatively spliced form of p85α from both mouse and rat cDNA libraries. This splice variant encodes a unique 5′-untranslated region, start codon, and 6-amino-acid aminoterminus followed by the carboxyterminal 418 amino acids of p85α. A corresponding exon is present within the p85α genomic locus.In vitrotranscription and translation of the splice variant cDNA generate a protein of approximately 45 kDa that is reactive with an anti-p85α antiserum. Northern blot analysis of mouse tissues reveals differential expression of full-length and alternatively spliced p85α, with the splice variant most abundant in the liver.