Histone-binding component that specifically recognizes H3 tails trimethylated on 'Lys-4' (H3K4me3), which mark transcription start sites of virtually all active genes. Interacts with H3K4me3 and to a lesser extent with H3K4me2. The PHD-type zinc finger mediates the binding to H3K4me3. Belongs to the Alfin family.