Human cytomegalovirus (HCMV) has been associated with several neoplastic diseases, including prostatic carcinoma, adenocarcinoma of the colon, cervical carcinoma, and Kaposi's sarcoma. The morphological transforming region II (mtrII) of CMV Towne has previously been localized to a 980 base-pair fragment containing three putative open reading frames (ORFs) of 79, 83, and 34 amino acids (aa). Homologous regions exist in strains Towne, AD169 and Tanaka. However, Tanaka mtrII has reduced transforming activity when compared to Towne and AD169. Interestingly, the 79 amino acid ORF present in both Towne and AD169 is truncated in Tanaka, indicating that it may be crucial for efficient transformation. Two functional promoters (P1 and P2) have been mapped within the Towne HCMV mtrII. The 79 amino acid ORF can be translated from the P1 transcript, while the 83 and 34 amino acid ORFs could be translated from either transcript. Similar functional promoter activities were detected within mtrII colinear transforming and non-transforming regions from HCMV strains AD169 and Tanaka, respectively. Neither promoter from strain Towne exhibited transforming activity. Combined with previous data, these data suggest that transformation by HCMV mtrII does not occur by promoter insertion but requires an intact 79 amino acid ORF.