UASt regulatory sequences are fused upstream of two open reading frames separated by stop codons (the upstream ORF is simply present to reduce translation of the downstream ORF). The downstream ORF encodes the PHD domain of Hsap\TAF3 (amino acids 856-929), which recognizes and binds H3K4me3 (His3 triply methylated at position K4), tagged at the N-terminal end with Tag:Dam.