In all the clinical cohorts examined hemizygous PTEN deletions were much more frequent than homozygous deletions in ER+/HER2- BC biopsies with PIK3CA activating mutations, although the precise frequency rates for hemizygous PTENdel might be sightly overestimated as a result of the sequencing technique used in each study. The gene discussed is ERBB2; the disease is breast cancer.