Wikipedia’s plot summary:
Holmes investigates when a wealthy philanthropist believes he was intentionally infected with an incurable illness — cerebral amyloid angiopathy (CAA); Holmes sends Joan to a suspicious dry cleaners to teach her deductive skills.
CAA is the result of an insoluble protein accumulating in the brain cells. In the episode this is caused by a drug-like molecule, which “specifically targets the gene”, which is impossible. If they’d only changed “gene” to the protein itself, it would have been much more believable.
Secondly, the detective receives the drug formula as a picture to his mobile and spends a night “solving the formula, determining where carbon, oxygen and nitrogen atoms go “. The formula on the screen briefly, and looks complete – anybody with a bit of chemistry training, for example his sidekick Joan Watson, would have told him the formula.
What I did like was the idea that blood test can be faked by mixing a “DNA-less” blood-like substance (for example, artificial plasma and anybody’s red blood cells – they don’t contain nucleus = DNA) with DNA synthesised to match a suspect 11 genetic markers (bits of his DNA). As nobody determines sequence of the whole length DNA, relying on the markers match, I cannot find a fault with the idea.