PIK3CA and esophageal squamous cell carcinoma: Footnote: ERRB3 v-erb-b2 erythroblastic leukemia viral oncogene homolog 3, ESCC esophageal squamous cell carcinoma, NRG1 neuregulin 1, PIK3CA amp PIK3CA amplified, PD progressive disease, PTEN phosphatase and tensin homolog, Q2W once every two weeks, QW once weekly, SCCHN squamous cell carcinoma of the head and neck, SD stable disease, UNK unknown; ∆Cq normalized gene expression.