PDCD1 and neoplasm: Checkpoint inhibition with αPD-1 (J43 and MuDX400) did not show differences in primary 4T1 tumor growth compared to controls up to the sixth injection at day 24, which is not surprising as in most mouse tumor studies, anti-PD-1 as a monotherapy yields modest to negligible effects due to the rapid growth of mouse tumor lines in vivo (Figure 2b).