ATR and ataxia telangiectasia: Although the biological importance of the binding of Vpr to CypA remains elusive, multiple functions of Vpr are connected to the two regions of the protein which bind cooperatively to CypA, including apoptosis [55-63], reverse transcriptase (RT) activity [36,64-66], replication of R5 tropic HIV-1 [38,67], nuclear localization of the protein [30,31,68-70], G2 cell cycle arrest [30-36,71] and binding of Vpr to DNA and RNA, which is linked to the ability of activating the ATR (ataxia-telangiectasia and Rad3-related) pathway leading to G2 arrest [72-75] (Figure 8).