Immunostaining to measure perforin expression by NK cells revealed significant differences between the three groups, F(2, 75) = 7.89, p = 0.001, η2 = .17, with the reduction in perforin expression restricted to the hip fracture patients with new onset depression compared to controls, p < 0.001 (Fig. 2c). Here, PRF1 is linked to major depressive disorder.