PDCD1 and cancer: The journey on how researchers identify the role of the microbiome in cancer therapy begun when mice with tumors but no gut microbiome showed a different response when treated with drugs like cyclophosphamide, oxaliplatin, cisplatin, and even anti-programmed cell death 1 protein (PD-1) immunotherapy (Iida et al., 2013; Viaud et al., 2013; Gopalakrishnan et al., 2018; Panebianco et al., 2018).