Myc is a highly unstable transcription factor that is destroyed by ubiquitin (Ub)-mediated proteolysis. We have previously identified an amino-terminal 'degron' within Myc that signals its destruction; this degron spans the transcriptional activation domain of Myc, and includes two highly conserved regions called Myc boxes I and II. We now report the identification of a second element--the D-element--which is also required for Myc proteolysis. The centrally located D-element is distinct from the PEST domain in Myc, but includes Myc box III, a third highly conserved region with no previously known function. We show that deletion of the D-element stabilizes the Myc protein without affecting its ubiquitylation, and report that the D-element and the degron act in a cell-type-specific manner to direct Myc proteolysis. These data thus demonstrate that Myc stability is regulated at both the ubiquitylation and postubiquitylation levels, and reveal that substrates of the Ub-proteasome system can be targeted for destruction differently in different cell types.