Working in his lab at the University of Colorado Medical School, Theodore was facing a serious issue. Since he successfully cloned HeLa cells in 1955, Theodore Puck had focused his attention on cytogenetic changes, but the high number and large size of human chromosomes didn't make his job any easier. Rodents were already in use in laboratories at that time, but could not help Puck. Mice have forty chromosomes. Rats have forty-two.