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 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