David A. Hungerford, 66, a researcher who was co-discoverer of the first visible genetic abnormality in cancer cells. Hungerford made the discovery in 1959 while working as a research fellow at the Institute for Cancer Research at Fox Chase in Philadelphia. Then a doctoral student in zoology, Hungerford and Peter C. Nowell found that a type of cancer of the blood called chronic granulocytic leukemia was linked to an arm of one chromosome. There are 23 pairs of rod-shaped chromosomes in human cells that carry genes. It was determined that the abnormality is not inherited but remains throughout the duration of leukemia, even when the disease is in remission. The abnormality is known as the Philadelphia chromosome after the city in which it was discovered. The discovery led other researchers to focus on genetic changes underlying some cancers, leading to the eventual identification of tumor genes called oncogenes. On Wednesday in Jenkintown, Pa., of lung cancer.