 The necessity of this system was established through a series of gene knockouts amongst "cryptochrome" ("Cry"), "period" ("Per"), and "Rev-erb" "--" core mammalian clock genes whose knockouts lead to arrhythmicity.The model that these researchers generated includes "Bmal1" as a driver of E-box mediated transcription, "Per2" and "Cry1" as early and late E-box repressors, respectively, as well as the D-box regulator Dbp and the nuclear receptor Rev-erb-α.The sequential inhibitions by "Rev-erb", "Per" and "Cry1" can generate sustained oscillations, and by clamping all other components except for this repressilator oscillations persisted with similar amplitudes and periods.