Updated project metadata. Deubiquitinases (DUBs) play a role in regulating protein degradation which is critical for maintaining protein homeostasis. The purpose of this study was to verify biological consequence of genetic depletion of USP21 in human cells on proteome changes and cellular function. Comparison of proteomic analysis of HAP-1 USP21 knockout and HAP-1 WT cells revealed that down-regulated proteins in HAP-1 USP21 KO cell are mostly engaged in mitochondrial function and ATP production. Cellular assays confirmed disturbance of mitochondrial metabolism and decrease in ATP production in HAP-1 USP21 knock-out cells.