Cyclic AMP-regulated phosphoprotein, 21 kDa, also known as ARPP21, is a human gene. Research published in March 2026 identified ARPP21 as one of the genes contributing to overall risk of developing amyotrophic lateral sclerosis (ALS). A new high-impact ARPP21 variant was identified (p.P747L). Furthermore, the p.P563L mutation, previously established as the cause of ALS in seven unrelated families from Spain, was confirmed to be associated with a more aggressive disease course.