SDC1 and Miyoshi myopathy: As MM is a tumor of Ab-producing PCs, precursors of PBs and PCs were identified by increased expression of CD38, decreased expression of CD45, and heterogenous (het) expression of CD19, CD20, CD27, and CD138 with surface membrane Igs (mIgs), such as IgA, IgG, and IgD (Figure 1C).