4G-shp53 cells can undergo secretory differentiation but do not form normal alveoli. Mice with human cells engrafted in the ducts were sacrificed on day 1 of lactation (n = 2). Histopathology (H&E stain) and immunohistochemistry for GFP show that human cells are present in the ducts but do not spread into the lactating alveoli. Immunohistochemistry for casein shows that some of the human cells in the ducts have undergone secretory differentiation. The structures in the middle of the image are mouse alveoli (open arrowheads); they do not stain for casein because the antibody is human specific. The structure on the right is a humanized duct containing cells that do not express casein (closed arrowheads). The structure on the left is a humanized duct containing cells that do express casein (arrows). Scale bars 200 μm.