MCF7 and SUM159 cells were cultured in mammosphere forming conditions. (A) Primary mammospheres were incubated with sulforaphane (0.5, 1, and 5 μM) or DMSO for 7 days. Sulforaphane treatment reduced the number of primary mammospheres. (B) Sulforaphane reduced the size of primary mammospheres(magnification × 100). The size of mammospheres was estimated using V = (4/3)π R3. (C) In the absence of drug, the 2nd and 3rd passages that were derived from sulforaphane-treated primary mammospheres yielded smaller numbers of spheres in comparison with control. Data are presented as mean ± SD(n = 3). *, P < 0.05; **, P < 0.01. SF = sulforaphane.