This family consists of human and simian mastadenovirus early E4 13 kDa proteins. Human adenovirus type 9 (Ad9) is unique in eliciting exclusively estrogen-dependent mammary tumours in rats and in not requiring viral E1 region transforming genes for tumorigenicity. E4 codes for an oncoprotein essential for tumourigenesis by Ad9.
Jiyao W et al.(2023). "The conserved domain database in 2023.",