To locate the E4 region on the genome of the HNF-70 prototype strain of the porcine adenovirus serotype 5 (PAdV-5), 4.5 kb at the extreme right end was sequenced. This area had 11 open reading frames (ORFs) on the left strand encoding more than 50 amino acids (aa). A Genbank homology search indicated that 8 of these ORFs were unique to PAdV-5. ORF5:253 aa, ORF6:245 aa and ORF11:146 aa showed similarities to known mammalian and avian adenovirus E4 ORFs. The putative E4 promoter sequences were also identified. Two TATA boxes, several GC boxes and two typical CCAAT boxes were located within this region. The presence of several potential poly-adenylation signals and the number of E4 transcripts detected by Northern blot analysis suggested the possibility of a complex mRNA transcription.