PCNA and neoplasm: Quantification of the proliferation marker protein Ki67 and the proliferating cell nuclear antigen (PCNA) signals in tumours showed considerable variation between, and even within, the specimens; however, both antibodies revealed a trend of more active tumour cell proliferation in PyMT;Col15a1−/− samples than in PyMT samples at weeks 6–9 (Supplementary Figure S1).