Why, you might ask? Easy enough to explain. The dead guy had been a politician who, when in a presidential race in the 70s, would blubber in newscasts. He drops out (like Muskie in that time frame, but wasn't afflicted), of course. It is later learned that he had early on-set Alzheimer's, but recovered and continued on to a brilliant time in Washington.
Enter the bad guy (well, we've seen him already...) with Binky's Brain. His intended use for the neurons is to use them (the dead guy recovered from Alz, of course) to concoct an instant cure for Alz. Did I mention that he runs a medical foundation? And that his son, some 40-ish years old, has early on-set Alz? You see where this is going. Goren and Eames have figured out the gag by now, with just a few minutes to go in the episode. They have the bad guy in the interrogation room, goading him to incriminate himself. And, of course this being teeVee, he does in spades. I can't find a transcript of the episode, so this is mostly correct from my lifelong lousy memory:
Bad guy: my team is working day and night to isolate the APOE4 gene and we'll have cure in no time! (Son is in the room, too)
Goren: so you found E4 in his brain?
Bad guy: yes, yes we have!!
Game over for the bad guy, of course. OK, so what's remarkable? How is it that a police procedural writer in 2002 knows about APOE4?
From the wiki
As of 2012, the E4 variant was the largest known genetic risk factor for late-onset sporadic Alzheimer's disease (AD) in a variety of ethnic groups.[64] However, the E4 variant does not correlate with risk in every population.Ten years after the episode, it's common knowledge.
So, when was APOE4 proposed as a factor in Alzheimer's? 1993 is the first report of correlation. But that's in the scientific literature. A quick look at the NYT indicates some references in the late 1990s. But that's some left wing fake news, after all.
In the end, watch high brow teeVee and learn something. Well... sort of. The dialog implies that finding E4 in the dead guy's brain is a good thing, the key to the bad guy's team to make a cure for the son's affliction. My memory of the exact dialog between Goren and the bad guy is fuzzy, so it's not clear how finding E4 (which is bad) in the recovered dead guy's brain is good for the bad guy's team making a cure. Poetic license someplace.