PDCD1 and neoplasm: In order to find other markers, which could allow to predict the clinical responsiveness to PD-L1/PD-1 checkpoint blockade, we also measured BM serum levels of different soluble factors known to regulate PD-L1 expression or to exert pro/anti-tumor activity in MM; we then analyzed the results in relation with PD-L1/PD-1 expression on PC and BM niche cells.