Both immunohistochemistry and Western blot analyses were used to identify the MHC isoforms in lion and caracal. Gel electrophoresis separated the MHC isoforms from caracal and lion muscle into four and three bands, respectively (Fig. 3A). The antibody 2F7 only recognized one band in all the species and corresponded to the migratory level of the human MHC IIa band (Fig. 3B). The MHC IIx antibody (6H1) only recognized one band for each of the species (Fig. 3C). However, these bands migrated differently from one another, suggesting a difference in molecular weight. The lion had the smallest MHC IIx isoform, followed by caracal and human. The position of the IIx isoforms was also confirmed when double labeling was performed with the MHC I (BA–D5) antibody (Fig. 3D). For all three species, only one MHC I band was identified and was similar in size at the lower migratory level.