Sodium butyrate induced cell cycle arrest in mammalian cells through an increase in p21 super(Waf1/Cip1), although another study showed that this arrest is related to pRB signaling. We isolated variants of HeLa cells adapted to growth in 5 mM butyrate. One of these variants, clone 5.1, constitutively expressed elevated levels of p21 super(Waf1/Cip1) when incubated in regular growth medium and in the presence of butyrate. Despite this elevated level of p21 super(Waf1/Cip1), the cells continue to proliferate, albeit at a slower rate than parental HeLa cells. Western blot analyses showed that other cell cycle regulatory proteins were not up-regulated to compensate for the elevated expression of p21 super(Waf1/Cip1). However, cyclin D1 was down-regulated by butyrate in HeLa cells but not in clone 5.1. We conclude that continued expression of cyclin D1 allowed clone 5.1 to grow in the presence of butyrate and elevated levels of p21 super(Waf1/Cip1).