SDC1 and monoclonal gammopathy: GADD45B was markedly upregulated in monoclonal CD138+ PCs from MM patients compared with monoclonal PCs from patients with monoclonal gammopathy of undetermined significance (MGUS), a premalignant condition (Kyle and Rajkumar, 2009), or healthy polyclonal PCs (Figure 1A), thus establishing a correlation between GADD45B mRNA expression and PC malignancy.