This excerpt taken from the PRXL DEF 14A filed Oct 26, 2007.
Restricted Stock Units
A participant will not have income upon the grant of a Restricted Stock Unit. A participant is not permitted to make a Section 83(b) election with respect to a Restricted Stock Unit Award. When the Restricted Stock Unit vests, the participant will have income on the date the stock is paid to the participant in an amount equal to the fair market value of the stock on such date less the purchase price, if any. When the stock is sold, the participant will have capital gain or loss equal to the sales proceeds less the value of the stock on the date the stock is paid to the participant. Any capital gain or loss will be long-term if the participant held the stock for more than one year and otherwise will be short-term.