This is almost an X-Men worthy mutant: the inability to feel pain. The researchers in this paper found three inbred families in Pakistan with the trait. The description is fascinating:

The index case for the present study was a ten-year-old child, well known to the medical service after regularly performing 'street theatre'. He placed knives through his arms and walked on burning coals, but experienced no pain. He died before being seen on his fourteenth birthday, after jumping off a house roof. Subsequently, we studied three further consanguineous families in which there were individuals with similar histories of a lack of pain appreciation, each originating from northern Pakistan and part of the Qureshi birdari/clan. All six affected individuals had never felt any pain, at any time, in any part of their body. Even as babies they had shown no evidence of pain appreciation. None knew what pain felt like, although the older individuals realized what actions should elicit pain (including acting as if in pain after football tackles). All had injuries to their lips (some requiring later plastic surgery) and/or tongue (with loss of the distal third in two cases), caused by biting themselves in the first 4 yr of life. All had frequent bruises and cuts, and most had suffered fractures or osteomyelitis, which were only diagnosed in retrospect because of painless limping or lack of use of a limb. The children were considered of normal intelligence by their parents and teachers, and by the caring physicians. One author saw and reviewed all six affected individuals and their families.

They then mapped the trait to mutations in SCN9A, a component of an ion channel expressed in pain-sensing neurons. Not only is this study really cool, but it could possibly lead to new painkillers or methods in anesthesia. Hooray for inbreeding!

ADDENDUM: Nick Wade has an article on the study.