Levels of PIK3CA E545K mutation (codon 545 GAG→AAG) were measured in normal breast, colonic mucosa, lung, and thyroid DNA samples, as well as in DNA isolated from ductal carcinomas, colonic adenomas, colonic adenocarcinomas, lung adenocarcinomas, and papillary thyroid carcinomas. Here, PIK3CA is linked to breast ductal adenocarcinoma.