CCR9 and obesity due to melanocortin 4 receptor deficiency: Despite being increased in the obese, the LEP-CXCL12-CCR4 network could play a negative role in breast cancer per se, whereas obesity might play a role in the functional switch of the LEPR-CCR9-CCL25 network, from promoting tumor immune infiltration in the non-obese, to ultimately favor tumor metastasis in the obese.