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 a single-chain variable fragment antibody ('15F11', see PMID:27534817) that binds (His3 that is mono-methylated at position K20), tagged with 2xTag:NLS(SV40-largeT) and with Tag:Dam.