ITGA2B and AIDS: This happens because conditioning or stratifying on an intermediate will adjust away the indirect effect of the treatment mediated by the cofounder, in this case CD41; and conditioning or stratifying on a collider creates a spurious association between the treatment and the unmeasured common cause that did not exist before conditioning (creating an open backdoor path AZT0 → CD41,... U→ AIDS), which is indicated by using dotted lines in the DAG of Figure 2D, leading to collider-stratification bias (Hernán et al., 2000; Cole et al., 2009; Ali et al., 2013).