PER2 and hydrops fetalis: To determine whether the altered HF cycle in DKO mice could be related to changes in molecular clock genes, we examined the expression levels of six CLOCK-regulated genes (Dbp, Npas2, Nr1d1, Per2, Cry2, and Clock) at PD50, when an upregulation of Dbp, Nr1d1, and Per2 normally occurs in WT mouse skin5.