CCND1 and Down syndrome: Chen et al. have reported that Dyrk1a can regulate the expressional balance of cyclin D1 and p21 to determine cell cycle entry in their studies of Down’s syndrome (Chen et al., 2013), and they concluded a p21-cyclin D1 signaling map, directing each cell to either proliferate or to follow cell cycle exit characterized by high or low cyclin D1 and p21 levels.