The critical role of leptin (rather than just obesity) in breast cancer progression was highlighted in an elegant experiment where obese ob/ob mice, which lack leptin, showed reduced mammary tumour outgrowth compared to increased tumour growth in obese db/db mice, lacking functional leptin receptor (LEPR) and hence having high circulating leptin levels [7]. Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.