Mother-to-infant transmission of HIV-1. Increasing evidence indicates that transplacental transmission of HIV-1 is the result of a complex interaction between genetic factors of the virus, mother, and infant. Maternal high virus load and low CD4⁺ lymphocyte count are most often associated with transmission with CCR5-using virus (non–syncytium-inducing [NSI], macrophage-tropic), most often transmitted even when CXCR4-using virus (syncytium-inducing [SI], T cell–tropic) is present in the mother. Host genetic factors of mother and infant are also important determinants of transmission. Polymorphisms in genes encoding for the chemokine receptor CCR5 in the infant can decrease the risk of transmission. Also, polymorphisms in genes encoding for cytokines and hormones such as LIF, hCG, and IL-4 likely contribute to the placental barrier to viral transmission. Figure adapted in part from ref.21.