“…The techniques of induced mutagenesis and modem molecular genetics have yielded remarkable results identifying genetic loci that affect pacemaker behavior in Drosophila and Neurospora (for review, see Hall and Rosbash, 1987). "Clock mutations" in rodents are also known: there are congenitally anophthalmic rats (Richter, 197 1;Ibuka, 1987) and mice (Scheuch et al, 1982;Gattermann et al, 1987) that do not entrain to light-dark (LD) cycles, mice with hypogenesis of the SCN that show disorganized circadian locomotor rhythmicity (Scheuch et al, 1982;Noguchi et al, 1986), mice that do not synthesize pineal melatonin (Ebihara et al, 1987) and golden hamsters with a mutation that shortens their pacemaker's free-running circadian period (Ralph and Menaker, 1988). Natural genetic differences (polymorphisms) manifested by inbred strains of rodents also suggest that genetic background affects circadian rhythmicity (e.g., Ebihara et al, 1978;Oliverio and Malomi, 1979;Possidente and Hegmann, 1980;Connolly and Lynch, 198 1;Peleg et al, 1982;Btittner and Wollnik, 1984;Beau, 1988) but more work is needed in this area.…”