Bortezomib down-regulates HLA class I in vivo on patient MM cells. Comparison of surface expression of HLA class I on the pretreatment and posttreatment of primary MM cells from 6 patients. The paired samples were cryopreserved, simultaneously thawed, and analyzed in the same experiment. PI-negative cells were analyzed for HLA class I expression. Open curves were before bortezomib. The black curves were 48 hours after a single dose of bortezomib 1.0 mg/m². The percentage of class I expression decrease was determined by 100 × (MFI of pretreatment sample − MFI of posttreatment sample)/MFI of pretreatment sample. Statistically significant difference of class I expression between the pre- and post- bortezomib treatment samples was present (P = .002, Student t test). The viability of pretherapy (▭) and after therapy () samples after thawing was similar.