UPDATE 1-Mylan seeks approval of generic of Merck's Vytorin

* Mylan expects 180 days exclusivity on generic

* Sued for patent infringement

NEW YORK Dec 22 (Reuters) - Mylan Inc (MYL.O) said on Tuesday that it had filed a U.S. application to market a generic version of Merck & Co's (MRK.N) big-selling and controversial Vytorin cholesterol drug.

Mylan, which did not say when it expected to launch the generic, said it had been sued for patent infringement by two Merck subsidiaries in connection with the filing.

Mylan, the world's No. 3 generic drugmaker, expects to qualify for 180 days as the lone marketer of generic Vytorin if its application is approved, the company said in a press release. Generic drugmakers generally receive this reward for challenging patents.

Sales of Vytorin, which combines an older statin drug with a newer cholesterol treatment called Zetia, have been pressured after clinical studies raised questions about its effectiveness. (Reporting by Lewis Krauskopf; Editing by Lisa Von Ahn)