A DNA sequence encoding the human CDK7 (P50613) (Ala 2-Phe 346) was fused with a polyhistidine tag at the N-terminus, constructed the plasmid 1; A DNA sequence encoding the human CCNH (P51946) (Tyr 2-Leu 323) was fused with a polyhistidine tag at the N-terminus, constructed the plasmid 2. A DNA sequence encoding the human MNAT1 (P51948) (Asp 2-Ser 309) was fused with a polyhistidine tag at the N-terminus, constructed the plasmid 3. The three plasmids were co-expressed and the heterotrimer was purified.