Enlarge By Shannon Stapleton, Reuters When international passengers enter the United States, they are fingerprinted. Here, a security officer is seen with a fingerprinting machine at JFK International Airport in New York in March 2008.

But what if you don't have fingerprints?

That was the dilemma faced by a Singapore cancer patient whose chemotherapy drug caused severe peeling of the skin on his hands and feet, which erased his fingerprints. His oncologist describes the case in a letter published online today by the Annals of Oncology.

The 62-year-old man was taking capecitabine, sold in the USA as Xeloda, for head and neck cancer that had spread to his bones, chest and abdomen (in the USA, Xeloda is approved for the treatment of breast and colorectal cancer that has spread). He developed hand-foot syndrome, a drug side effect that causes the skin on the hands and feet to peel.

After taking capecitabine for more than three years, the man, who wasn't identified by doctors, flew to the USA to visit relatives. He was detained at the U.S. airport by Customs and Border Protection officers for four hours because they couldn't detect his fingerprints, his doctors, from the National Cancer Centre Singapore, write.

Finally, the officers were satisfied that he wasn't a security threat and allowed him to enter the country. They told him to travel with a letter from his oncologist explaining his lack of fingerprints.

Two years ago, Spanish cancer doctors reported a similar story about a 39-year-old flight attendant detained for several hours at a U.S. airport until her doctor faxed an explanation that the capecitabine she'd been taking for breast cancer had erased her fingerprints.

Many other drugs can cause hand-foot syndrome, but there is little information about whether they lead to fingerprint loss, Su-Pin Choo, one of the co-authors of the new letter, said in an e-mail.

"Hand-foot syndrome is more common with capecitabine than with most other drugs," Choo wrote. Fingerprint loss probably is also related to how long a patient takes a drug that causes hand-foot syndrome, he said, and he added that patients receiving a continuous infusion of 5FU, a common cancer drug, also should consider carrying a letter attesting to that if they travel to the USA.

In the world, an estimated one in 50 people lack matchable fingerprints. "We have standard operating procedures that take that into account," says the Department of Homeland Security's Anna Hinken. She says Customs and Border Protection officers decide whether to admit such people on the basis of other physical and behavioral traits.