IFNB1 and infection: To further investigate whether there is evidence of selection imposed by the fetal immune system, not only on whether transmission arises but also on the quality of viruses transmitted from the mother, we compared VRCs, IFN-α and IFN-β IC50 values, pVL, and absolute CD4 counts for the mothers of males (MoM), for the mothers of females (MoF), and for male and female infants, segregated by timing of maternal infection (Fig. 4).