CD44 and breast cancer: To confirm whether the combination of sorafenib and radiation could more effectively suppress mammosphere formation than single treatments such as reduction of CD44+CD24−/low cells from co-treated breast cancer cells, cells were cultured in radiation (10 Gy), sorafenib (5 μg/ml), radiation + sorafenib (10 Gy + 5 μg/ml) or DMSO in ultra-low attachment plates and then cultured to the secondary passage in the absence of sorafenib and radiation under normoxic and hypoxic conditions.