S9-CMC1 TFA is a covalent peptide lysine-specific demethylase 1 ( LSD1 ) inhibitor with an IC 50 value of 2.53 μM. S9-CMC1 TFA specifically recognizes Cys360 in the enzyme-active region. S9-CMC1 TFA inhibits LSD1 activity, increasing H3K4me1 and H3K4me2 levels, leading to G1 cell cycle arrest and apoptosis and inhibiting cell proliferation. S9-CMC1 TFA significantly inhibits tumor growth in A549 xenograft animal models.