Stem cell biologists in the US have been waiting for this day for almost 8 years. With the stroke of a pen, President Barack Obama this morning removed the limits on federal funding of research on human embryonic stem cells (ESCs) imposed by his immediate predecessor.

"What happened today is huge," says Kevin Wilson, director of public policy at the American Society for Cell Biology. "We've gone from having a small number of cell lines eligible for federal funding to having at least a few hundred."

ESCs can develop into any other cell type found in the body, and so have huge potential for use in medicine, to repair lost or damaged tissues. But some people object to their use on moral grounds because the creation of an ESC line usually involves the destruction of a pre-implantation embryo, just a few days old.

Existing cell lines
As a compromise, President George Bush announced on 9 August 2001 that federal funds would only be made available for research on cell lines derived prior to that date.

From the start, critics argued that the policy would impair progress in research. And as scientists scrutinised the list of approved lines, it emerged that there were many fewer than the 60 claimed in Bush's announcement.

"It really has limited people," says Sean Morrison, who heads the Center for Stem Cell Biology at the University of Michigan in Ann Arbor.

Separate labs
For US biologists who work on human ESCs, Obama's executive order means the end to an administrative nightmare that has seen them set up separate lab space and equipment to ensure that federal dollars are not inadvertently used for research on non-authorised cells.

"The press has just been here taking pictures of us ripping stickers off our incubators," Evan Snyder of the Burnham Institute for Medical Research in La Jolla, California, told New Scientist.

The removal of these restrictions may now convince more researchers to start working on human ESCs. "My own research is on adult stem cells," says Elaine Fuchs of Rockefeller University in New York City. "We haven't worked with human embryonic stem cells in part because of the hurdles involved."

Federal funding restrictions
Obama's executive order gives the National Institutes of Health 120 days to establish new guidelines for research on human ESCs and authorises the agency to back this work "to the extent permitted by law".

This frees biologists to work with a wide range of human ESCs - including cell lines created with state and private funding.

But researchers are not expected to be able to use federal grants to create new cell lines. This is because of a 1996 law called the Dickey-Wicker amendment, attached to the bills approving the NIH's budget, which bans funding for research that involves the destruction, injury or death of a human embryo.