Previous work has reported the presence of an abnormally high percent of B cells (10%–30%) in PBMC from patients with MM when detected with anti-CD19 mAbs B4 or FMC63 (Bergsagel, Masellis, Szczepek, Mant, Belch, and Pilarski, 1995; Szczepek, Seeberger, Wizniak, Mant, Belch, and Pilarski, 1998) but a low % when detected with the Leu12 mAb (Kay et al. 1997). The gene discussed is CD19; the disease is Miyoshi myopathy.