The spontaneous dwarf rat (SDR) was found in a laboratory colony of Sprague Dawley rats in 1977. It is a recessive mutation that causes them to have reduced GH or Growth Hormone which causes them to be up 40-75% smaller than their normal-sized counterparts, and in fact, a little larger than some fancy English mice. Dwarf rats have been found to be resistant to some cancers, as scientists have studied the effect of chemically induced cancer on dwarf rats and found dwarfs do not develop cancerous tumors like typically sized rats due to their lack of Growth Hormone.