As far as I know, selenocysteine is the only reason you need selenium in your diet (which you almost certainly get enough of; the requirement is vanishingly small, 100 micrograms/day). It is a member of the same group as oxygen. As you go down a group, things change in subtle ways. Sulfur is a less-electronegative, bigger, more polarizable, more nucleophilic, stinkier oxygen. Similarly, selenium is all these things, but more so!
It is the nucleophilicity that is so important; it is found in the amino acid selenocysteine - that is, the selenium analogue of the sulfur amino acid cysteine (or the oxygen amino acid serine).
Selenocysteine plays a role in thioredoxin reductase, the reaction of which with curcumin may have chemotherapeutic applications.
One of the methods for offsetting selenium toxicity is exposure to arsenic, however, exposure to selenium also offsets arsenic toxicity. I guess two wrongs do make a right.
http://www.jstor.org/pss/3428469
Good timing on this post as FDA just yanked a dietary supplement with 200-400X the recommended selenium content. The website for the product (Total Body Formula) is down so I don't know what form of selenium was in the supplement.