TH and attention deficit-hyperactivity disorder: Our previous study showed that miRNA let-7d was abnormally expressed in the prefrontal cortex (PFC), the major affected brain area in ADHD, in spontaneously hypertensive rats (SHRs), a well-established animal model of ADHD with similar therapy response to human [6], and the abnormal expression of let-7d was associated with the regulation of tyrosine hydroxylase [7].