Shortest Domain Names

Of nowhere I got this thought to investigate what might be the shortest functioning domain name in the world. The only possibility is domain names with a letter or a digit.

As most of the single digit and single letter domain names are reserved by the Internet Assigned Numbers Authority, I was not sure about the existence of single character domain names. But I was pleasantly surprised to know that there are actually few:

G (Google China)
Q (Qwest)
X (Pay Pal)