To quantitatively assess PU.1 expression in synovial tissue we performed RT-PCR on synovial tissue lysates, which revealed that RA patients with follicular synovitis have significantly less expression of PU.1 compared to RA patients with diffuse synovitis (P=0.02, Mann-Whitney U-test) (Supplementary Fig. 5). Here, SPI1 is linked to synovitis.