Complex sentence: MHC molecules were identified and named after their role in transplant rejection between mice of different strains, though it took over 20 years to clarify MHC's role in presenting peptide antigens to cytotoxic T lymphocytes (CTLs).
Split into simpler sentences: MHC molecules were identified and named after their role in transplant rejection between mice of different strains. It took over 20 years to clarify the role of MHC in peptide presentation to cytotoxic T lymphocytes (CTLs).