CDKN1A and rheumatoid arthritis: We then examined whether this was attributable to an effect on cyclin-dependent kinase inhibitors, such as p21, by incubating RA-FLS with 1 μM MPT0G009 or 3 μM SAHA for 24 h and assessed the expression of p21 by flow cytometry (Figure 1h) or western blot (Supplementary Figure 2c).