Chaetocin is a specific inhibitor of the histone methyltransferase (HMT) SU(VAR)3-9 with an IC₅₀ of 0.6 μM for SU(VAR)3-9. It also inhibits thioredoxin reductase (TrxR) with an IC₅₀ of 4 μM. IC50 & Target: IC50: 0.6 μM (HMT)^[1], 4 μM (TrxR)^[2] In Vitro: Chaetocin is initially isolated from the fermentation broth of chaetomium minutum and belongs to the class of 3-6 epidithio-diketopiperazines (ETPs). The IC₅₀ for SU(VAR)3-9 is 0.6 μM and acts as a competitive inhibitor for S-adenosylmethionine. Chaetocin inhibits the human ortholog of dSU(VAR)3-9 with a similar IC₅₀ value of 0.8 μM. It inhibits other known Lys9-specific HMTs such as mouse G9a and Neurospora crassa DIM5 with a higher IC₅₀ values of 2.5 and 3 mM, respectively^[1]. Chaetocin inhibits TrxR1-initiated turnover of the synthetic substrate DTNB in a cell-free assay in a dose-responsive manner with an IC₅₀ of about 4 μM^[2]. In Vivo: SL-2 Drosophila tissue cells are cultured in the presence or absence of the inhibitor. Chaetocin has a toxic effect on cells grown in culture. Toxicity is highly dependent on the initial cell density when chaetocin is added to the culture. The number of H3 molecules dimethylated at Lys9 (H3K9me2) is markedly reduced when cells are grown in medium containing 0.5 μM chaetocin after 5 d. Histones isolated from cells treated with 0.1 μM and for a shorter time also shows a drop in Lys9 methylation, but not as strongly as with the higher concentration^[1].