The disclosure provides nucleic acids including the polynucleotide sequence of the human MAFbx core promoter involved in muscle specific expression. Also provided are reporters operably linked to a polynucleotide sequence including MAFbx transcription regulatory sequences, and constructs including polynucleotides that encode reporters and other polynucleotide sequences operably linked to the MAFbx core transcription regulatory sequence. Systems for identifying agents that inhibit muscle loss and/or increase muscle mass or tone are also provided.