In agreement with functional redundancy of NPAS2 and CLOCK [20,27], only double deficiency of CLOCK and NPAS2 in mice results in arthropathy [28](E.A. Yu and D.R. Weaver, personal communication) similar to arthropathy developed upon BMAL1 deficiency [10] CLOCK is the major interacting partner of BMAL1 [12]. This evidence concerns the gene NPAS2 and arthropathy.