Covalent modifications of histones play important roles in the regulation of chromatin dynamics and gene activity. Until recently, it was believed that methyl groups could not be removed from histones; as such, the discovery of the first demethylases opens a novel era in understanding how chromatin dynamic is regulated and shows that active demethylation is linked to both transcriptional repression and activation. During androgen-dependent gene activation, specific demethylases are involved in the control of gene expression. These new findings represent a milestone in elucidating the function of demethylases in gene expression. Furthermore, they show that active demethylation of repressive histone marks is a hallmark in the control of specific gene expression.