PLK1 and neoplasm: Furthermore, Takahashi et al (2003) demonstrated that, in human colorectal carcinoma, PLK1 protein overexpression was directly related to the proliferating cell nuclear antigen (PCNA) LI, as well as some other parameters reflecting the biological aggressiveness of the tumour such as Dukes' classification, primary tumour invasion, lymph node metastasis, and aberrant p53 expression.