K562-MERT colonies are smaller in the presence of tamoxifen. A mixture of K562 and K562-MERT cells were cultured in methylcellulose in the absence (A-B) and presence (C-D) of 1 μM tamoxifen. After 14 days, the resulting colonies were examined by phase (A,C) and fluorescent (B,D) microscopy using an Olympus IMT-2 microscope (Olympus, Melville, NY), 100 × magnification, 10 ×/0.25 objective lens, 10 × ocular lens, and an Olympus C-35AD-4 camera. The bright and dim colonies in panels A and C contain K562-MERT and K562 cells, respectively.