To test the specificity of the PG/G1-reactive ACPA+ RA serum (ACPA+#9) against the citrullinated form of PG, we treated the rhG1 domain of PG [22] with PAD4 enzyme, and then subjected these native and citrullinated rhG1 proteins to Western blotting with anti-hG1 mAb, ACPA+#9 serum, and an Ab to chemically modified citrulline. This evidence concerns the gene PKD1P1 and rheumatoid arthritis.