B2M and monocytic leukemia: This T cell clone (DGB129) recognized cell lines constitutively displaying surface MR1 (CCRF-SB lymphoblastic leukemia cells, or THP-1 monocytic leukemia cells; Figure 1A) or A375 melanoma cells (A375-MR1) transfected with an MR1-β2m fusion gene construct (Lepore et al., 2014) (Figure 1A) in the absence of any exogenously added antigens (Figure 1B).