PDCD1 and lymphopenia: In another study, lymphopenia (<1000 cells/μl) upon initiation of PD-1 inhibitor nivolumab was not associated with poorer survival in patients with head and neck squamous cell carcinomas (n=100), but persistent lymphopenia under nivolumab was associated with poorer overall survival in multivariate analysis (HR 3.96, 1.19-13.17, p=0.034) (141).