Genetics and hormones play a role in the development of Hori’s Naevus.
In normal skin, melanin-producing pigment cells (melanocytes) are present only in the upper layer of skin (the epidermis) and hair follicles. However, when these cells are present in the underlying area of skin (the dermis) it causes a condition known as dermal melanocytosis. Dermal melanocytosis is a spectrum of conditions and includes Mongolian spots, naevus of Ota, Hori’s Naevus and naevus of Ito.
When melanin is present more deeply in the skin, it results in the perceived colour changes from black-brown to blue-grey.