Effect of HOXB13 on androgen-independent prostate cancer cell growth: S4 and Tet-on cells were grown under androgen-rich conditions (A, C) or androgen-deprived conditions (B, D) for up to 6 days in 100 nM doxycycline. HOXB13-suppressed cells (1-3 and 4-1) with scrambled control cells were also grown under similar conditions (E, F). The cells were stained with MTT reagent and measured at an optical density at 570 nm. The * in the diagram, determined by a two-tailed Student's t test, corresponds to the difference between the two groups (p < 0.05). In E and F, repeated-measures ANOVA revealed very significant interaction effect between groups and days (p < 0.0001).