Mammary carcinomas that develop in NRL-PRL females exhibit diverse histotypes, and express varying levels of ERα. The proportion of cells containing immuno-detectable ERα was quantitated as described in Materials and methods, and shown related to tumor histotype. Each box represents a single tumor that arose in a different NRL-PRL nonparous female. The dashed line marks 10% ERα+ cells, a common clinically used threshold. (Tumor histotypes: GL, glandular; Pap, papillar; Adsq, adenosquamous; C-S, carcinosarcoma).