while the per3 gene is a paralog to the per1 and per2 genes, studies in animals generally show that it does not contribute significantly to circadian rhythms. functional per3-/- animals experience only small changes in free-running period, and do not respond significantly differently to light pulses. per1-/- and per2-/- animals experience a significant change in free-running period; however, knocking out per3 in addition to either per1 or per2 has little effect on free-running rhythms. furthermore, per1-/-per2-/- mice are completely arrhythmic, indicating that these two genes have much more importance to the biological clock than per3.