Laboratory of Population Genetics, National Cancer Institute, Gaithersburg, MD 20877, USA.|Laboratory of Population Genetics, National Cancer Institute, Gaithersburg, MD 20877, USA.|Laboratory of Population Genetics, National Cancer Institute, Gaithersburg, MD 20877, USA.|Laboratory of Population Genetics, National Cancer Institute, Gaithersburg, MD 20877, USA.|Laboratory of Population Genetics, National Cancer Institute, Gaithersburg, MD 20877, USA.