The National Enquirer has learned that Dana Reeve — widow of Superman star Christopher Reeve — has been diagnosed with lung cancer.

Sources said that Dana, 44, who took up the battle for stem cell research begun by her heroic husband, is receiving treatment.

Dana has also limited her public appearances as she battles cancer.

On July 13, she cancelled an appearance in Washington, D.C., for “family reasons.”

She was to appear with actor Michael J. Fox to support a bill to lift restrictions on stem cell research.

In addition to coping with the tragedy of Christopher’s death last October 10, Dana lost her mother Helen, 71, earlier this year. She died on February 10 from complications following ovarian cancer surgery.

A source said Dana, who was not a smoker, is being supported by family and friends.

The tragedy comes as Dana has become a strong advocate for breakthrough medical solutions as dedicated as her brave husband, who was left a quadriplegic after a horseback riding accident in 1995.

Since Christopher Reeve’s death at 52, Dana, who lives with their 12-year-old son Will in Pound Ridge, N.Y., has taken up chairmanship of the Christopher Reeve Paralysis Foundation.

The organization funds research that develops treatments for paralysis and other disorders.

Dana proudly launched the foundation’s Superman Tag program, which raises money through the sale of dog tags engraved with the Superman “S” shield and the foundation’s new tagline, “Go Forward.”

As the cancer she must now battle struck, Dana was reinvigorating her singing career.

In June, she performed a cabaret act at Feinstein’s, a club in New York’s Regency Hotel.

Numerous calls placed by The National Enquirer for comment to the Reeve foundation were not returned.