To investigate the underlying mechanisms, a qPCR analysis of mammary glands and tumour tissue revealed that the <i>n</i>-3 PUFA diets downregulated the expression of pro-tumourigenic immune markers, including CD206 and F4/80 in the mammary glands and the cannabinoid receptor CB2 in tumours, compared to the <i>n</i>-6 PUFA diets.<h4>Conclusions</h4>These findings indicate that the presence of dietary <i>n</i>-3 PUFA plays a key role in modulating mammary tumour development, which may be further influenced by the underlying <i>n</i>-6 PUFA background. This evidence concerns the gene MRC1 and breast cancer.