PDCD1 and metastatic melanoma: Although there is still no evidence that CD44-HGF affects the response to anti-PD-1/anti-PD-L1 therapies, a recent study identified types of immune cells known as classical monocytes (CD14+CD16–HLA-DRhi) in the peripheral blood as potential biomarkers for responses to anti-PD-1 immune checkpoint therapy in metastatic melanoma (Goswami et al., 2018).