Mice homozygous for the mutation showed 27.3 hour periods, but eventually lost all circadian rhythmicity after several days in constant darkness. That showed that "intact CLOCK genes" are necessary for normal mammalian circadian function .
In mice with the mutation, a 27.3-hour cycle was observed initially; however, they ultimately lost their circadian rhythms within a few days of being kept in complete darkness. This finding suggests that "intact CLOCK genes" are necessary for proper mammalian circadian function.