In this study, we showed that the analysis of immune infiltrates in mouse models of cancer are likely to be confounded by 1) significant experimental variability between and within experiments, leading to difficulties in interpreting complex immune data; 2) analysis of individual “known” cells, such as CD3+ CD4+ cells, based on predetermined phenotype that may miss large populations of unconventional cells, such as CD3+ CD4- CD8- cells; and 3) differences in tumour type or injection site that can have large effects on immune infiltrate composition. Here, CD4 is linked to cancer.