CD4 and asthma: Interestingly, the CD4+/CD8+ ratio of T cells in peripheral blood is more correlated in MZ twins than in DZ twins [Yokoyama & Akiyama, 1995; Evans et al., 1999; Hall et al., 2000], and between 65% and 84% of the variation in CD4+/CD8+ ratio is explained by genetic factors [Evans et al., 1999; Hall et al., 2000], indicating a substantial influence of genetics on this trait with an important role in the atopic lymphocyte switch in asthma.