Scientists analysing the bird flu virus which is infecting people in Turkey say it is a particularly nasty form, but one which has been seen elsewhere.
Humans contract the virus from infected birds
The analysis of a sample from one Turkish case showed a genetic change which has been seen in previous human cases in Hong Kong and Vietnam.
However, the London-based team stress the alteration does not make the virus more likely to pass between humans.
So far, 18 people in Turkey have been infected and three have died.
Experts from the National Institute of Medical Research in London are analysing samples on behalf of the WHO.
They have looked at two so far.
One has shown a particular mutation of the hemaglutinin gene.
The same mutation has also been seen in human viruses during the 2003 outbreak in Hong Kong and last year's outbreak in Vietnam.
Because this particular mutation has been seen before in humans, scientists say there is no reason for these findings to increase concern.
The tests also show that the virus was certainly caught from birds, and not from other humans.
Sir John Skehel, director of the Medical Research Council's National Institute for Medical Research suggested that the change is something that can happen once the virus gets into a person to make it more efficient within that individual.
The gene sequences of the viruses from Turkey also indicated that they were sensitive to the antiviral drugs Tamiflu and an old form of anti-viral medication called amantadine, said the scientists.
They say this might suggest a combination of drugs would be most effective at tackling the virus if it does mutate into a form which can spread between humans.