Skunky Beer

What causes skunked beer? From The Washington Post:

Chemists at the University of North Carolina and Ghent University in Belgium found that when exposed to light, the alpha acids in hops break down into free radicals that then react with sulfur-containing proteins to make a chemical called 3-methyl-2-butene-1-thiol, which is virtually identical to the principal constituent of skunk juice.

When they say skunked, they mean skunked.