Abnormal generation of neurotoxic amyloid-beta peptide (A beta) 42/43 species due to mutations in the catalytic presenilin 1 (PS1) subunit of gamma-secretase is the major cause of familial Alzheimer's disease (FAD). Deeper mechanistic insight on the generation of A beta 43 is still lacking, and it is unclear whether gamma-secretase modulators (GSMs) can reduce the levels of this A beta species. By comparing several types of A beta 43-generating FAD mutants, we observe that very high levels of A beta 43 are often produced when presenilin function is severely impaired. Altered interactions of C99, the precursor of A beta, are found for all mutants and are independent of their particular effect on A beta production. Furthermore, unlike previously described GSMs, the novel compound RO7019009 can effectively lower A beta 43 production of all mutants. Finally, substrate-binding competition experiments suggest that RO7019009 acts mechanistically after initial C99 binding. We conclude that altered C99 interactions are a common feature of diverse types of PS1 FAD mutants and that also patients with A beta 43-generating FAD mutations could in principle be treated by GSMs.