Protein L-Isoaspartate-O-Methyltransferase (PCMT1) is a crucial enzyme involved in the repair of damaged proteins. It catalyzes the methylation of abnormal L-isoaspartyl residues and, to a lesser extent, D-aspartyl residues in proteins, which arise from the spontaneous deamidation and isomerization of asparagine and aspartate residues over time. This post-translational modification serves as a key step in the repair process, allowing these damaged proteins to be either restored to their normal structure or targeted for degradation, thereby maintaining protein homeostasis within cells. As PCMT1 has the function of cell proliferation , we measure the activity of recombinant human PCMT1 by the ability of the protein to stimulate MCF-7 cells proliferation. MCF-7 cells were seeded into triplicate wells of 96-well plates and allowed to attach, replaced with various concentrations of recombinant human PCMT1. After incubated for 72h, cells were observed by inverted microscope and cell proliferation was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10 µl of CCK-8 solution was added to each well of the plate, then the absorbance at 450 nm was measured using a microplate reader after incubating the plate for 1-4 hours at 37 ℃. Cell viability was assessed by CCK-8 assay after incubation with recombinant human PCMT1 for 72h. The result was shown in Figure 1. It was obvious that PCMT1 significantly increased cell viability of MCF-7 cells. The ED50 of recombinant human PCMT1 is 2.059 μg/ml.