TRIM63 and Atrophy: In particular, transcriptional upregulation of several E3 ubiquitin ligases, including Fbxo32 (atrogin-1/MAFbx), Trim63 (MuRF1), Fbxo30 (MUSA), and Fbxo21 (SMART) has been reported to occur in atrophy and is considered responsible for poly-ubiquitin tagging and proteasomal degradation of target proteins during muscle wasting4,5.