Although the target of immunomodulatory drugs (IMiDs)—cereblon (CRBN)—is not an essential gene in multiple myeloma (MM) cells, genomic defects in CRBN, including mutation, copy number loss, and a specific exon-10–deleted splice transcript variant (henceforth called CRBN-Del-Exon10), increase in IMiD-resistant relapsed and refractory MM (RRMM) patients (Ito et al., 2010; Lopez-Girona et al., 2012; Thakurta et al., 2014; Kortum et al., 2016; Franssen et al., 2018; Gooding et al., 2021). This evidence concerns the gene CRBN and plasma cell myeloma.