The role of HIV replicative fitness in perinatal transmission of HIV. 
Perinatal transmission of Human immunodeficiency virus (HIV), also called mother-to-child transmission (MTCT), accounts for 90% of infections in infants worldwide and occurs in 30%-45% of children born to untreated HIV-1 infected mothers.  Among HIV-1 infected mothers, some viruses are transmitted from mothers to their infants while others are not.  The relationship between virologic properties and the pathogenesis caused by HIV-1 remains unclear.  Previous studies have demonstrated that one obvious source of selective pressure in the perinatal transmission of HIV-1 is maternal neutralizing antibodies.  Recent studies have shown that viruses which are successfully transmitted to the child have growth advantages over those not transmitted, when those two viruses are grown together.  Furthermore, the higher fitness is determined by the gp120 protein of the virus envelope.  This suggests that the selective transmission of viruses with higher fitness occurred exclusively, regardless of transmission routes.  There are many factors contributing to the selective transmission and HIV replicative fitness is an important one that should not be neglected.  This review summarizes current knowledge of the role of HIV replicative fitness in HIV MTCT transmission and the determinants of viral fitness upon MTCT.