CD8A and Menkes disease: In contrast, our MHC-congenic lines, which share the same genetic background (presumably including TCR and Rfp-Y loci) but carry MHC haplotypes linked to different levels of MD resistance, showed mild (5–15%) differences in TCR usage in the CD8+ T cell population only, indicating that MHC class I, but not necessarily MHC class II in this model, shapes different T cell repertoires during thymic selection that are recognizable at the bulk (gene family) expression level.