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CD45.1 is a protein isoform of CD45, also known as PTPRC, which is a receptor-type tyrosine phosphatase expressed on all hematopoietic cells except erythrocytes and platelets. This type I transmembrane protein plays a crucial role in the activation processes of B and T cell receptors, as well as in thymic selection. CD45.1, along with CD45.2, is commonly used in adoptive cell transfer experiments to distinguish donor cells from host cells due to their allelic differences. Mice carrying the CD45.1 allele, such as C57BL/6J, are widely used in research for this purpose. CD45.1 is also used in studies to understand the rejuvenating effects of young blood on aged tissues. In heterochronic parabiosis models, where the circulatory systems of old and young mice are connected, CD45.1 and CD45.2 genotypes are used to track the cells of old and young mice, respectively.