Degradation of many short-lived cellular proteins such as the transcription factor MyoD occurs via the ubiquitin-proteasome pathway. MyoD, similar to many rapidly degraded regulatory factors, interacts with several high affinity binding partners, including members of the Id (inhibitors of DNA binding) family. Following transfection to HeLa cells, Id1 is localized to the nucleus and rapidly (t1/2 ∼ 1 h) degraded via the ubiquitin-proteasome system. Mutagenesis of lysine residues within the putative nuclear localization region (amino acids 68-82) directs Id1NLS to the cytoplasm yet confers an increased rate of degradation (t1/2 ∼ 0.5 h). Id1 in which all lysine residues were mutagenized to alanine (lysineless Id1) was also rapidly degraded (t1/2, ∼ 0.6 h). Addition of a Myc6 tag to the N terminus of lysine-less Id1 markedly stabilized Id1 (t 1/2 > 10 h) and suggests degradation via the N terminus-dependent pathway. Co-transfection of MyoD with Id1 or Id1NLS increases Id1 or Id1NLS within the nucleus and markedly reduces the rate of Id1 or Id1NLS degradation. These results thus demonstrate that in vivo MyoD modulates the rate of Id1 degradation and suggest a dynamic interplay of these factors.