NIH 3T3 fibroblasts that express little detectable levels of the MIF binding receptor CD74 but do express the signalling component CD44 were used for the identification of interacting proteins that are shared by MIF and D-DT. Endogenously expressed MIF and D-DT were tagged at the C-terminus using a short sequence that is recognized and biotinylated by the bacterial biotin ligase BirA (Efficient biotinylation and single-step purification of tagged transcription factors in mammalian cells and transgenic mice. Proc. Natl. Acad. Sci. U. S. A. 100, 7480–5) as described in detail for MIF Filip, A. M. et al. (2009). Ribosomal Protein S19 Interacts with Macrophage Migration Inhibitory Factor and Attenuates Its Pro-inflammatory Function. J. Biol. Chem. 284, 7977–7985). After pulling down biotinylated proteins from stable NIH 3T3 clones that express tagged MIF or D-DT and BirA ligase with streptavidin agarose, bound proteins were separated by SDS-PAGE and identified after elution and tryptic digestion by mass spectrometry in order to characterize the MIF and D-DT interactomes.