Background: Being an informal caregiver (IC) of a cancer patient is often associated with psychological distress. We have recently, in a randomized controlled trial (RCT), demonstrated efficacy of Emotion Regulation Therapy for ICs (ERT-C), evidenced as lower levels of psychological distress. Such efficacy demonstration is important, but a crucial step in improving treatments for the IC population is the identification of moderators (i.e., for whom the treatment works) and mediators (i.e., the drivers of the detected effect).