A study in Scientific Reports has used isotransplantation to examine the effect of mouse breeding nuclei in prostate cancer development and intratumoural macrophage populations, illustrating the importance of understanding mouse models to explore the tumour immune microenviroment (TIME). TRAMP-C1 cells were implanted into C57BL/6J mice from two breeding nuclei (nA and nB) and tumour growth period and intratumoural macrophage profile were measured. After 69 days, 54% of nB mice showed tumour implantation, whereas 100% of nA mice showed implantation after 28 days. A significantly increased M2 macrophage profile was observed in the TIME from both mice groups, but they were not significantly different from each other. However, nB tumours exhibited approximately twice the population of M1 macrophages (11–27%) of nA (4–15%) and fewer non-polarized macrophages; M1:M2 average ratio was 1:8 for group nA and 1:4 for nB. These differing tumour progression and macrophage populations in mice from the same substrain illustrate the importance of animal source renewal for controlling murine cancer model variables, especially in TIME research.