BTK and Miyoshi myopathy: Besides CD138 expression, functional markers such as SP staining and aldehyde dehydrogenase activity (Aldefluor assay), mRNA expression of induced pluripotent stem cells genes (i.e. OCT4, NANOG, SOX2), drug resistance and chromosomal instability genes (i.e. NEK2, BTK, RARα2), BCRP, and/or ALDH1A1 expression has been used to identify stem-like MM cells [7, 8].