Restoring arginine levels, even in moderate amounts comparable to a couple of over-the-counter tablets, appeared to revive expression of the genes needed to make MHC-1. First author Qiushuang Wu, a postdoctoral researcher in the lab, examined how shifts in amino acid availability affect gene expression. In cell culture experiments, declining arginine levels reduced the abundance of 414 proteins. Without sufficient arginine, ribosomes stall while assembling MHC-1 because the protein contains many sites that require the amino acid. Arginine levels naturally decline with age, potentially diminishing the immune system's ability to recognize abnormal or infected cells.