W. Heston giving an intravenous injection to mice. Heston studied pulmonary tumors and heredity in mice with an attempt to localize tumor susceptibility to specific genes. Medical researchers use mice for numerous reasons. Mice are small, require little food or housing space, have consistent disease manifestations, have good-sized litters of offspring, are easy to handle, and can be readily shipped from breeding facilities to research locations. Their use as cancer models has provided exceptional insight into the biology of human cancers, and, more recently, their genetics. No photographer credited, dated 1944.