Instead, we found that the original phenotypic differences in the two effector populations were still present in memory populations even 5 days after rechallenging tertiary memory cells (i.e. quaternary memory): memory cells derived from minimal effectors expressed less KLRG1 and more CD27 (Fig. 4B), but expression levels of KLRG1 and CD27 had no impact on the quality of the memory response as measured by the ability to proliferate and clear a viral infection (Fig. 4B, C). Here, CD27 is linked to viral infectious disease.