This family consists of Homo sapiens and simian mastadenovirus early E4 13 kDa proteins. Human adenovirus 9 (HAdV-9) is unique in eliciting exclusively estrogen-dependent mammary tumours in Rattus spp. and in not requiring viral E1 region transforming genes for tumorigenicity. E4 codes for an oncoprotein essential for tumourigenesis by Ad9 [1].