Furthermore, if there is an unmeasured common cause (U) of both the time-varying confounder (CD41) and the outcome (progression to AIDS) as in DAG of Figure 2C, the time-varying confounder (CD41) is also a “collider” on the path AZT0 → CD41 ← U → AIDS (the arrows from U and CD40 collide on CD41). Here, CD40 is linked to AIDS.