Cyclic AMP-regulated phosphoprotein, , also known as ARPP-21, is a human gene. Research published in March 2026 identified ARPP-21 as one of the genes contributing to overall risk of developing amyotrophic lateral sclerosis (ALS). A new high-impact ARPP-21 variant was identified (p.P747L), and variant p.P563L was confirmed to be associated with more aggressive disease progression.