Is Pfizer (NYSE: PFE) folding on making acquisition deals? That seems to be a trick question as the drug giant announced the purchase of FoldRx yesterday -- not to be confused with Amicus Therapeutics (Nasdaq: FOLD), which goes by the ticker FOLD.

Both Amicus and FoldRx study protein misfolding, which can cause diseases. In FoldRx's case, Pfizer is getting tafamidis meglumine, a drug that treats a rare disease called TTR amyloid polyneuropathy (ATTR-PN). FoldRx has already submitted the drug for approval in Europe and is discussing with the Food and Drug Administration what additional studies, if any, need to be done to gain U.S. approval.

The companies didn't give terms for the deal, but did say it was structured based on milestones. Both of those are positives for Pfizer's investors since it means the deal isn't material to Pfizer's financials, and it's not on the hook for as much if the drug approval doesn't pan out.

If approved, the lead drug will be a nice addition, but Pfizer is likely pursuing FoldRx's pipeline and expertise in rare diseases. Drugs that treat orphan diseases usually fetch high prices, which makes up for the smaller potential patient base -- ATTR-PN affects "at least 8,000 patients worldwide." Not very many.

Pfizer seems increasingly enamored of drugs that treat rare diseases. It recently licensed Protalix BioTherapeutics' (NYSE: PLX) Gaucher disease drug, which will compete against Genzyme's (Nasdaq: GENZ) Cerezyme and Shire's (Nasdaq: SHPGY) Vpriv if the Food and Drug Administration signs off on the drug.

Given the increased interest, might Pfizer be interested in outbidding sanofi-aventis (NYSE: SNY) for Genzyme? Possible, but that would lead to a conflict of interest between Genzyme's and Protalix's drugs. It might make more sense to buy BioMarin Pharmaceutical (Nasdaq: BMRN), which is a bit smaller, but equally as focused on orphan drugs, if Pfizer wants to continue to expand in that direction.

Either way, it's clear there will be another acquisition. Pfizer isn't going to fold anytime soon.

Matt Koppenheffer hates this stock, but he isn't willing to short it either.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.