Washington ? That sticky gunk coating Alzheimer’s patients’ brains gets all the notoriety, but another culprit is gaining renewed attention: Protein tangles that clog brain cells and just might determine how fast patients go downhill.

Nobody knows what causes Alzheimer’s, although the disease’s hallmark plaque — that gooey stuff called beta-amyloid — is the main suspect. Yet repeated attempts at anti-amyloid treatments have failed, the latest disappointment last month when Eli Lilly & Co. abandoned an experimental drug that wound up doing harm, not good.

Now comes a different clue: A second protein called tau seems to signal how aggressive the mind-robbing disease will be. Researchers discovered that patients with mild Alzheimer’s and high levels of tau also harbored a genetic alteration that in turn predicted they would worsen faster.

That suggests if scientists could figure out how to lower tau levels, it might slow dementia, says senior researcher Alison Goate of Washington University in St. Louis.

More than 5 million Americans are estimated to be living with Alzheimer’s, as many as half in the disease’s early stages. The only available medications temporarily ease symptoms but don’t slow the disease.

How quickly a loved one will deteriorate is a big question for families struggling to plan for care — and Goate’s work is a first step at identifying genetic markers to help predict how long someone may function independently and when they might require a nursing home.

Don’t look for a genetic test for tau any time soon. This is first-step research that needs to be validated by other laboratories, and Goate says it’s likely just one genetic marker among many to be discovered. Already, a handful of drug companies are focusing on the protein.