CD34 and infection: Transcription is regulated by cell-specific factors, while DNA replication is most likely regulated by species-specific factors, and for this reason, JCV has a limited host replication tropism; thus, in vitro cell transfection with JCV DNA results in the infection of only those cell types known to allow infection in vivo (as tonsillar stromal cells, B cells, and CD34+ hemopoietic cells).