The development of new and improved vector systems is central for realization of new concepts for gene therapy. The tropism of human cytomegalovirus (CMV) for hematopoietic progenitor cells and the large genome size (230 kbp) that offers a unique cloning capacity make this virus a promising vector candidate for gene transfer into hematopoietic cells and for therapy of congenital and acquired diseases of the hematopoietic system. Recently, we cloned the CMV genome as a bacterial artificial chromosome (BAC) in Escherichia coli and established efficient mutagenesis procedures for CMV - a prerequisite for vector construction. Here, we report on the construction of a recombinant GFP virus that will be used to re-evaluate the tropism of CMV and to monitor gene transfer into target cells. Further goals of CMV vector development are the evaluation of the cloning capacity and the construction of replication-deficient vectors.