PDCD1 and colorectal cancer: In other tumor entities such as lung cancer, colorectal cancer and melanoma, PD-L1 expression was shown to have both positive and negative prediction value which may be due to the following reasons [22]: Primer-based detection of PD-1 and PD-L1 has technical issues, such as variable primer sequences, tissue preparation, processing variability or different statistical cut-offs resulting in misleading expression status.