Mutagen X (Yeah, I picked it just for the name)

As with chloroform, the carbon source is usually humic acids.

I'm a water engineer (BS in Chem Eng, MS in Environmental Eng). I'm not sure, but I would guess that the carbon comes from humic or fulvic acids, i.e. NOM (natural organic matter) that is naturally present in surface water, and occasionally groundwater. It's not a single simple molecule, it's a whole family of complex organic soup. I personally haven't heard of Mutagen X, though there are all kinds of disinfection byproducts (DBPs) out there. Chloroform is the most common. THMs (trihalomethanes, of which chloroform is obviously one) and HAAs (haloacetic acids) are the ones that are regulated by the SDWA (safe drinking water act). Is that enough TLAs for you? (three letter acronyms)

Yeah, I've read the "humic acid" explanation, and maybe I'm showing my ignorance, but can't we do a little better? Five-membered lactenones always remind me of ascorbate. Anyone have any idea what the real precursor is? It can't be that diverse a range of substrates for transformation by what amounts to chlorine radical.

This would be a good theme for a science-fiction story, IMHO...

Looks like the carbon could have come from an isoprene, but those can come from practically anywhere.

I remember the story in Peru where they were worried about trihalomethanes causing cancer in drinking water. So they stopped chlorinating the water for a while, and many died from water-borne bacterial diseases. I'd rather drink THM's than, say, cryptosporidium, etc.

Compare Mutagen X, structure and steric bulk, with that of Vioxx,

http://www.worldofmolecules.com/drugs/Rofecoxib.png

A little lab work toward the intersection would be interesting. Furanones can be scccharide degradation products. To what does Splenda pyrolyze at cooking, baking, and crust-browning temps?