A novel Human Cytomegalovirus vector system is provided. The vector system is characterized by an amplicon vector, having an HCMV derived origin and cleavage packaging signal, and a helper virus function. The vector system is capable of infecting a wide range of host cells, including human CD34.sup.+ hematopoietic progenitor cells. Methods, including therapeutic and prophylactic methods, for introducing heterologous polynucleotide sequences into cells using the vector system are also provided.