Philadelphia chromosome

noun

Phil·​a·​del·​phia chromosome ˌfil-ə-ˈdel-fē-ə- 

: an abnormally short chromosome 22 that is found in the hematopoietic cells of persons affected with chronic myelogenous leukemia and lacks the major part of its long arm which has usually undergone translocation to chromosome 9

A bone marrow test the next day revealed a genetic abnormality called the Philadelphia chromosome that is the signature of chronic myelogenous leukemia, or C.M.L., a blood cell cancer that in the last decade has been transformed from ultimately fatal to nearly always treatable …

—Jane E. Brody, The New York Times

Scientists have continued to hammer away at the so-called Philadelphia chromosome, a shortened chromosome 22. They now know that it results when chromosomes 9 and 22 exchange genetic material.

—Nathan Seppa, Science News