This study tested the 5-HTTLPR gene as a moderator in the relation between maternal unresponsiveness and child externalizing symptoms in a disadvantaged, predominantly Black sample of two hundred and one 2-year-old children and their mothers. Using a multimethod, prospective design, structural equation model analyses indicated that maternal unresponsiveness significantly predicted increases in externalizing symptoms 2 years later only for children possessing the LL genotype. Moderation was expressed in a “for better” or “for worse” form hypothesized in differential susceptibility theory. In examining why the risk posed by maternal unresponsiveness differed across the 5-HTTLPR polymorphism, mediated moderation analyses showed that children's angry reactivity to maternal negativity partly accounted for the greater susceptibility of homozygous L carriers to variations in maternal unresponsiveness.