Positive selection is what changes double positive T-cells into single positive T-cells. Positive selection occurs in the cortex and negative in the medulla.
I learned this in lecture, and I do not have access to UWorld or AMBOSS yet so I included this research article which speaks briefly on this in the intro. Was not sure where else to find this information. But I have pasted the quote below and the link below that:
"Complex proteins (MHC) can undergo positive selection, resulting in maturation and commitment to either the CD4 or CD8 lineage, depending on whether the selecting MHC was class II or class I, respectively (5). Positive selection also leads to the relocalization of thymocytes from the cortex to the medulla, and the majority of medullary thymocytes exhibit a CD4+CD8− or CD4−CD8+ “single positive” SP phenotype."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4938245/
