Abstract The processing of the amyloid precursor protein (APP) and the sterol regulatory element binding protein show remarkable analogies. Following a first lumenal cleavage, both proteins undergo a cleavage within the transmembrane domain by enzymatic activities named γ-secretase and S2P, respectively. We analyzed the processing of APP in the mutant Chinese hamster ovary (CHO) cell line M19 which lacks the S2P gene encoding for a putative metalloprotease. In these cells, we were not able to detect any β-amyloid production from endogenous or transiently overexpressed APP, although the transport of APP along the secretory pathway, its processing by α- and β-secretase, as well as its secretion were normal. This strongly suggests that the γ-secretase cleavage in M19 cells is severely impaired.