PD-L1 expression on tumour cells has been recognised as a negative prognostic marker as it correlates with the shorter 5-year survival rate, the Clark level, the presence of lymph node metastases [164], the mitotic rate, the Ki67 index, metastases, the presence of ulceration [165], and perineural invasion [166]. The gene discussed is MKI67; the disease is neoplasm.