Enteric adenovirus types 40 and 41 (Ad40 and Ad41), representing subgenus F, differ from all other human adenoviruses by being so fastidious that productive replication does not occur in conventional established cell lines. They are dependent of the Ad5 early regions E1A and E1B since they can not grow in HEK cells, only in 293 HEK cells transformed by Ad5 E1. The overall genetic organization of Ad41 E1A is similar to the E1A region of other characterized human adenoviruses but it is slightly shorter, comprising 1350 bp. The inverted terminal repeat (ITR) at the 5' end of both Ad40 and AD41 consists of 163 nucleotides, being similar to the ITR of Ad12 (subgenus A) and longer than the ITRs of adenoviruses of subgenera B, C, and E. The early mRNA products (12 and 13 S) can be translated into a 222-amino acid (aa) and a 251-aa tentative protein, respectively. In a comparison of the Ad41 251-aa protein with corresponding peptides of Ad12, Ad7, Ad5, and Ad4, three conserved amino acid sequences CS1-CS3 can be found. In the second conserved domain CS2, which is particularly acidic, the homology is very high within all five serotypes compared. Only one among eight conserved amino acids differs in the Ad41 251-aa protein. Within CS1 and CS3 which exhibit a hydrophilic and a hydrophobic character, respectively, the amino acid composition of the Ad41 protein is less conserved than the corresponding regions in all other analyzed adenovirus types. Ten of 16 conserved amino acids in CS1 are shared by Ad41 and 18 of 23 conserved amino acids in CS3 are shared by Ad41.