A adenovirus type 5 host range mutant (hr440) has been isolated which is defective in a splicing event required to generate the middle-sized mRNA from early region 1A. This defect has been ascribed to two adjacent nucleotide changes which lie five and six nucleotides from the 5' splice site for this mRNA (Solnick, Nature 291:508-510, 1981). One of these changes introduces an amber codon into the reading frame of the largest region 1A mRNA, resulting in the production of a truncated polypeptide. Like other region 1A mutants, hr440 is defective in the production of mRNA from early regions 2 and 3, but hr440 is unusual in that transcription from regions 1B and 4 is normal. Furthermore, although region 1B expression is unaffected, hr440 does not transform baby rat kidney cells. Therefore, expression of early region 1B is insufficient for transformation, eliminating the possibility that region 1A is required only to induce such expression.