Accessory mammary tissue is a common congenital defect located along the embryonic ‘milk lines’. In ancient times it was regarded as a proof of divinity in men and fertility in women. More recently it has been considered an atavistic trait without any clinical significance, according to the evolutionary theory by Darwin.

During the past 30 years, however, the survey of accessory mammary tissue has drawn considerable interest with respect to possible associated conditions, most notably kidney and urinary tract malformations.

In 1996 we first reported the original occurrence between supernumerary nipple and Becker nevus, in extension of the Becker nevus syndrome. Based on the findings of our studies, we report here an overview of these novel observations and discuss some possible embryologic backgrounds involving specific developmental fields (anlage/skin), and genetic relationships (paradominance) linking these cutaneous defects.