CD274 and neoplasm: In addition, some patients with PD-L1 negative tumours have been noted to respond to anti-PD-1/PD-L1-directed treatments, with some of these responses being durable, thus limiting PD-L1 expression as an exclusionary biomarker.61 Evaluation of PD-L1 protein levels on tumour samples has been fraught with difficulties with regard to assay types and reproducibility.