EN2 and Parkinson disease: Most interestingly, mice heterozygous null for En1 and homozygous null for En2 (En1+/-;En2-/-; from here on EnHT), which are viable and fertile, exhibit a slow progressive loss of nigral dopaminergic neurons within the first two months after birth, resulting in diminished storage and release of dopamine in the striatum and in PD-like motor deficiencies [14].