This excerpt taken from the NDAQ 10-K filed Mar 14, 2005.
QQQ Lists on Nasdaq
On December 1, 2004, the Nasdaq-100 Index Tracking Stock (QQQ) transferred its listing to Nasdaq from Amex. Nasdaq and Amex reached a mutual decision to transfer the QQQ earlier than the contractual date of June 2005. It trades under the symbol QQQQ.