Genzyme ( GENZ) Monday said federal regulators have asked for additional information about its proposed acquisition of Ilex Oncology ( ILXO).

Genzyme said in late February that it would pay $1 billion in an all-stock deal for the San Antonio, Texas-based Ilex which specializes in cancer therapies.

The Cambridge, Mass-based biotech company recently reiterated its belief that it can close the Ilex deal during mid-2004 but, in a statement Monday, said the Federal Trade Commission's request extends the waiting period under which it can review the transaction under antitrust regulations.

Shares in Genzyme shed 40 cents, or 0.9%, to $46.06, while Ilex shares fell 39 cents, or 1.6%, to $23.81. Ilex shares were valued at $26 at the time of Genzyme's offer.