A team at Roche’s biotechnology unit Genentech in California compared all the genetic changes in a single patient’s lung tumor with healthy tissue from the patient, a 51-year-old man who had smoked an average of 25 cigarettes per day for 15 years before the tumor was removed.

What they found were as many as 50,000 genetic mutations.

“Fifty thousand is a huge number. No one has ever reported such a high number,” said Zemin Zhang of Genentech, whose findings appear in the journal Nature.

“This is likely associated with the smoking history of the patient. It is very alarming,” Zhang said in a telephone interview.

Smoking is the biggest single cause of lung cancer, and studies suggest mutations occur with each cigarette smoked.