In BIcγ1 and BIcγ1-derived mice, lethal MM was uniformly preceded by an MGUS-like stage from 6 months of age, characterized by minimal BM infiltration of oligoclonal GFP+CD138+B220−sIgM− PCs that moderately secreted class-switched immunoglobulins into the serum (Fig. 1k,l and Extended Data Fig. 3d,e). Here, SDC1 is linked to Miyoshi myopathy.