Washington, March 31 : American scientists have created a "switch" that allows mutations or light signals to be turned on in muscle stem cells to monitor muscle regeneration in a living mammal.

The research team, including 2007 Nobel Prize winner Mario R. Capecchi, say that their breakthrough may lead to a genetic switch, or drugs, that allows people to grow new muscle cells to replace those that are damaged, worn out, or non-functional.

Washington, March 31 : Researchers at St. Jude Children''s Research Hospital have found that eye cells believed to be retinal stem cells are actually normal adult cells, and thus therapies to restore vision in people with retinal degeneration should involve other types of stem cells.

Scientists have always believed that retinal stem cells may form the basis for treatments to restore sight to millions of people with blindness caused by retinal degeneration, provided they can be obtained.

Miami  - Rafael Nadal and Andy Murray were both pushed by aggressive opponents Monday before the pair of leading seeds constructed survival scenarios to reach the fourth round at the Miami Masters.

Number 1 Nadal had fought off a spirited challenge from Portuguese challenger Federico Gil for a tight, 7-5, 6-3 victory.

"No opponent is easy, especially if you are not playing your best tennis," said Nadal. "Everything was more difficult today. I didn't play very well, and I had chances to lose the first set."

Washington - Barack Obama's newly named special envoy for Sudan, retired Air Force major general Scott Gration, met the US president, Sudan human-rights advocates and members of Congress Monday before travelling this week to the region.

"I can't think of somebody who is better equipped to travel to Africa, the continent where he grew up, and communicate to Sudan a couple of important points," said Obama, who has pledged to keep pressure on Khartoum to end the conflict in the Sudanese region of Darfur, describing the humanitarian crisis there as "urgent."

New York  - US stocks tumbled Monday amid concerns over the fate of the struggling car industry, which will be given a last chance to restructure operations or lose government support.

President Barack Obama said that General Motors Corp and Chrysler LLC had so far failed to prove that they can survive. He called for a series of changes in the two companies' outdated business models and forced out long-time GM chief executive Rick Wagoner.

Washington  - President Barack Obama on Monday approved a sweeping expansion of government land protections, safeguarding about 800,000 new hectares of parks, rivers, forests and mountains across the United States.

The legislation, which is a collection of about 150 smaller bills that had been in the works for years, was approved earlier this month with broad bipartisan support by both chambers of Congress.

The bill designates new wilderness areas across nine US states, including mountains in California, Virginia and Colorado, protecting those regions from any kind of commercial development.