APOBEC3G and viral infectious disease: Interactions between cellular restriction factors (such as A3G and SAMHD1), their cognate viral proteins (e.g., HIV Vif and SIV Vpx) and the host CRL (CRL5 and CRL4) are enticing targets for anti-viral drug development as augmenting the restriction factor (or inhibiting the viral protein) would allow the hosts’ immune system to control and counteract viral infection.