Substance P (SP), a neuropeptide that, along with the less well characterised neurokinin-A (NKA), is encoded by the human TAC1 gene (Tac1 in rodents) and is expressed in both the medial and central amygdala where it modulates anxiety levels (Ebner et al., 2008; Singewald et al., 2008; Zhao et al., 2009). The gene discussed is TAC1; the disease is Anxiety.