RETREG1 and Parkinson disease: FAM134B isoform, FAM134B-2, is known to be induced under amino acid starvation and regulated by transcription factors such as MEF2D, NR4A1, BACH1, and ZBTB10.38 Among these, NR4A1 was upregulated by MSCs in both our cellular and animal PD models, followed by an increase in FAM134B.