Familial hypercholesterolemia is a genetic disorder that results from various gene mutations, primarily within the LDL receptor (LDLR). Approximately 50% of the LDLR mutations are defined as class 2 mutations, with the mutant proteins partially or entirely retained in the endoplasmic reticulum. To determine the degradation pathway of the LDLR class 2 mutants, we examined the effects of inhibition of several potential pathways on the levels of the wild-type LDLR and its four representative class 2 mutants (S156L, C176Y, E207K, and C646Y) stably expressed in Chinese hamster ovary (CHO) cells. We found that proteasome inhibitors MG132 and lactacystin blocked the degradation of the LDLR mutants, but not that of the wild-type LDLR. Treatment of CHO cells with these proteasome inhibitors led to a significant accumulation of the mutants at steady state. Furthermore, cell surface levels of the LDLR mutants were significantly increased upon inhibition of the proteasome degradation pathway. In contrast to the proteasome inhibitors, inhibitors of trypsin-like proteases, chymotrypsin-like proteases, and lysosomal pathway inhibitors did not affect the levels of the LDLR mutants. Taken together, these data demonstrate that the proteasome is the principal degradation pathway for LDLR class 2 mutants.