PIK3CA and esophageal squamous cell carcinoma: Moreover, our previous mutational signature analysis showed that hotspot mutations (c.1624G>A:p.Glu542Lys, c.1633G>A:p.Glu545Lys) of PIK3CA were significantly enriched in ESCC tumors that had an apolipoprotein B mRNA-editing enzyme catalytic (APOBEC) signature in both cohort #1 and cohort #3, which implicates APOBEC activity as a common key driver of PIK3CA mutagenesis in ESCC patients from Northern or Southern populations in China [10].