At approximately day 1

Basal cells in the breast are positive for “basal cell markers” (karatin 14 and smooth muscle).

Reference [PubMed Link]

Howard BA, Gusterson BA. 2000. Human breast development. Journal of Mammary Gland Biology and Neoplasia 5(2):119-137.

At approximately day 1

Two cell populations can be distinguished in the epithelium: the inner luminal cell population which is separated from the basement membrane by an outer basal (myoepithielial) layer. The epidermal transforming growth factor TGFα is highly expressed, suggesting proliferation.

Reference [PubMed Link]

Howard BA, Gusterson BA. 2000. Human breast development. Journal of Mammary Gland Biology and Neoplasia 5(2):119-137.