This excerpt taken from the SIRI 10-K filed Mar 1, 2007.
Transfer Restrictions. Shares of Restricted Stock and Restricted Stock Units may not be sold, assigned, transferred, pledged or otherwise encumbered, except, in the case of Restricted Stock, as provided in the Plan or the applicable Award Agreements. Certificates issued in respect of Shares of Restricted Stock shall be registered in the name of the Participant and deposited by such Participant, together with a stock power endorsed in blank, with the Company. Upon the lapse of the restrictions applicable to such Shares of Restricted Stock, the Company shall deliver such certificates to the Participant or the Participant's legal representative.