The phosphatidylinositol-3-kinase pathway is one of the most commonly altered molecular pathways in invasive breast carcinoma, with phosphatidylinositol-3-kinase catalytic subunit (PIK3CA) mutations in 25% of invasive carcinomas. Ductal carcinoma in situ (DCIS), benign papillomas, and small numbers of columnar cell lesions harbor an analogous spectrum of PIK3CA and AKT1 mutations, yet there is little data on usual ductal hyperplasia and atypical ductal and lobular neoplasias. We screened 192 formalin-fixed paraffin-embedded breast lesions from 75 patients for point mutations using a multiplexed panel encompassing 643 point mutations across 53 genes, including 58 PIK3CA substitutions. PIK3CA point mutations were identified in 31/62 (50%) proliferative lesions (usual ductal hyperplasia and columnar cell change), 10/14 (71%) atypical hyperplasias (atypical ductal hyperplasia and flat epithelial atypia), 7/16 (44%) lobular neoplasias (atypical lobular hyperplasia and lobular carcinoma in situ), 10/21 (48%) DCIS, and 13/37 (35%) invasive carcinomas. In genotyping multiple lesions of different stage from the same patient/specimen, we found considerable heterogeneity; most notably, in 12 specimens the proliferative lesion was PIK3CA mutant but the concurrent carcinoma was wild type. In 11 additional specimens, proliferative epithelium and cancer contained different point mutations. The frequently discordant genotypes of usual ductal hyperplasia/columnar cell change and concurrent carcinoma support a role for PIK3CA-activating point mutations in breast epithelial proliferation, perhaps more so than transformation. Further, these data suggest that proliferative breast lesions are heterogeneous and may represent non-obligate precursors of invasive carcinoma. Keywords: atypical ductal hyperplasia; breast carcinoma; columnar cell change; PIK3CA; usual ductal hyperplasia The phosphatidylinositol-3-kinase pathway is one of the most commonly mutated pathways in invasive breast carcinoma. Activating mutations in the phosphatidylinositol-3-kinase catalytic subunit (PIK3CA) are present in B25% of invasive carcinomas and are present in an even higher percentage (on the order of 40%) of low-grade estrogen receptorpositive carcinomas (luminal A intrinsic subtype). 1-10 PIK3CA mutations cluster in 'hotspots' in exon 9 (helical domain, E542K and E545K) and exon 20 (kinase domain, H1047R/L). [1][2][3][4][5][6][7][8][9][10] In addition, this pathway is activated by mutations in the plekstrinhomology domain of AKT1 in B5% of breast carcinomas, by the loss of the phosphatase PTEN (phosphatase and tensin homolog on chromosome 10), or rarely by amplification or alterations in other regulatory subunits, and is a target of active drug development. [8][9][10][11] In recent large studies of estrogen receptor-positive tumors, the presence of activating PIK3CA hotspot point mutations has been paradoxically associated with more favorable outcome as compared with breast carcinomas with wild-type PIK3CA. 3,12,13 We and other groups have previously s...