To validate the specificity of DT2216, we used the stable isotope labeling with amino acids in cell culture (SILAC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomics to analyze the changes in proteins in WI-38 normal human diploid fibroblasts after DT2216 and DT2216NC treatment. The results show that DT2216, but not DT2216NC, reduced the levels of BCL-XL, but none of other proteins were significantly affected by either agent, demonstrating that DT2216 is a specific BCL-XL PROTAC.