The amino-terminal tails of histone proteins are subject to a variety of post-translational modifications; addition or removal of these 'marks' facilitates gene activation or silencing. Here, a mechanism is defined that modulates the activity of the dual-specificity histone demethylase LSD1 during androgen-dependent transcription. Androgen-dependent signalling through protein kinase C beta I leads to phosphorylation of a histone amino acid, which prevents demethylation of an adjacent amino acid by LSD1