CD27 and synovial sarcoma: Thus, we first used 4-color flow cytometry to analyze the frequency of naïve (CD3−CD19+CD27−IgD+), class-switched memory B cells (CD3−CD19+CD27+IgD–) and unswitched memory B cells (CD3−CD19+CD27+IgD+) in patients with SS (Fig. 1A).