Forward genetic approaches led to the discovery of the first mammalian gene, Clock, to be identified at the molecular level in the core circadian clock. The persistence of clock function when only one paralog is removed is fairly common in the molecular clock system; however, few genes seem to be completely redundant when genome-wide transcript and transcription technologies were implemented it became clear that between 2% and 10% of all genes were expressed with a near 24-h rhythm, and each of these is not considered as a core clock gene, where the transcription/translation feedback loop is the basis of many observed behavioral and cellular rhythms, there are some molecular circadian rhythms that can oscillate independently of transcription. Investment into development of drugs that can treat sleep, mood, and metabolic disorders using the gears of the molecular clock are well underway.