The two canceled studies were of people with early-stage, sporadic Alzheimer’s, but Roche will continue a separate trial of the drug involving people with a mutation that puts them at risk of inheriting Alzheimer’s.

The setback adds to the long list of failed Alzheimer’s treatments—Fortune noted more than half a dozen flameouts in 2018, alone. An older study tallied only one approved drug from 244 tested for Alzheimer’s or dementia between 2002 and 2012. As a result, investors are growing ever more skeptical that an effective Alzheimer’s treatment can be produced, even after a drug achieves a positive trial result.

One of the leading theories of dementia and Alzheimer’s is that the buildup of certain proteins called amyloid plaques and tau tangles in the brain interfere with normal memory formation and other higher-level functions. As the diseases progress, the buildups deprive people of cognitive functions in an irreversible backward march until they are left with child-like abilities and die.

Roche hoped that crenezumab would be able to stop this at source by binding to the amyloid plaques and restricting their ability to spread.

Frustrating progress has led researchers of all stripes to propose alternative causes such as cyanobacteria and gum bacteria. Some big pharma companies, including Pfizer, have abandoned the field altogether. The solution, if there is one, may instead need to come from some unexpected quarter, like so many other medical advances.