Obesity has been linked to breast cancer progression but the underlying mechanisms remain obscure. Being overweight or obese for a woman at the time she is diagnosed with breast cancer is linked to a high risk of recurrence regardless of treatment factors. In rodents, high body weight is also associated with increased incidence of spontaneous and chemically induced tumors. To study the complex interaction between the mammary epithelia and the microenvironment, with a focus on the mechanism underlying the role obesity plays in the regulation of the cancer stem cell traits and the development of mammary cancer in vivo, we have established a diet-induced obesity (DIO) rat model of breast cancer (Chang et al., 2015).