CD274 and Merkel cell skin cancer: To date, only few studies have analyzed the PD-L1 expression in CTCs of patients with different solid tumors such as breast cancer [28], head and neck cancer [29,30,31], prostate cancer [32], gastrointestinal cancers [19,32], bladder cancer [33], melanoma [34], Merkel-cell carcinoma [35], and NSCLC [18,20,36,37,38,39,40,41,42,43,44].