While the 2-h leisurely forest walk generated an increased NK cell number and activity and expression of anti-cancer proteins in a series of field experiments conducted by Li et al. [1], we could not detect such explicit changes following our 2-h forest walking program; however, activation of all examined cell subsets (by increased expressions of CD69 and NKG2D and decreased expression of TIM-3) and increased expressions of perforin and granzyme B by NKbright cells were revealed in late spring. Here, PRF1 is linked to cancer.