SLC7A1 and rheumatoid arthritis: LC–MS/MS was used to determine L-arginine concentration in the culture supernatant of RA FLSs and revealed that its concentrations were increased in the culture supernatant of RA FLSs by silencing CAT-1 with siRNA under both normoxic and hypoxic conditions, indicating that CAT-1 is the primary transporter for L-arginine in RA FLSs (Fig. 3c, d).