From CNN.com:
Shares of Sirius Satellite Radio (down $2.22 to $6.79, Research) fell 20 percent and topped the Nasdaq's most-active list after Smith Barney cut its rating to "sell" from "hold," citing valuation. Shares have been on a tear since October, when shock jock Howard Stern said he would leave Viacom's Infinity Broadcasting to host a show for the paid radio station, starting in 2006.