Perforin (Prf1) is a molecule expressed by cytotoxic lymphocytes that are used in killing by transiently inducing pore formation target cells and allowing Grzs to be internalized and induce cell death.28 Although perforin can act with Grzs to induce cytotoxic events, the effects of GrzM in UC appeared to be perforin independent, since the Prf1-deficient mice displayed no enhancement of colitis post DSS challenge (Supplementary Figure 2E). The gene discussed is PRF1; the disease is colitis.