PTEN and breast cancer: This knowledge is of clinical importance because in existing clinical trials targeting the PI3K pathway in breast cancer, a fresh biopsy is usually not required to test PTEN expression, and available archival tissue is acceptable for IHC to assess PTEN loss.11,13, , –16 In addition, advances in molecular testing have provided robust information on genomic alterations of cancer biomarkers in many patients diagnosed with breast cancer.