Ribophorin I (RPN1) is a key component of the 19S regulatory particle, which forms part of the 26S proteasome, a large multicatalytic complex responsible for the degradation of ubiquitinated proteins in eukaryotic cells. As a non-ATPase subunit within the regulatory particle, RPN1 plays a critical role in facilitating the recognition, unfolding, and translocation of target proteins into the 20S core particle, where proteolysis occurs. To test the effect of RPN1 on cell proliferation, MDA-MB-231 cells were seeded into triplicate wells of 96-well plates and allowed to attach, replaced with various concentrations of recombinant human RPN1. 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 RPN1 for 72h. The result was shown in Figure 1. It was obvious that RPN1 significantly decreased cell viability of MDA-MB-231 cells. The ED50 of recombinant human RPN1 is 2.5 μg/mL.