Smallest data URI image possible for a transparent image

I'm using a transparent 1x1 image with a background image, to be able to use sprites and still provide alternative text for some icons.

I want to use a data URI for the image to reduce the number of HTTP requests, but what would be the smallest possible string to produce a transparent image?

I realize I could use data URI:s for the actual images instead of sprites, but it's easier to maintain when everything is kept in the CSS instead of scattered around.