RXRB and infection: There are at least two genes in the Tir1 region that are directly involved in cholesterol metabolism (Abcg1 which participates in cholesterol efflux from the cell and Rxrb which regulates Abc mediated efflux[22]) but cholesterol plays an important role in the response to infection and Tnfa which is also in the Tir1 region has been shown to regulate cholesterol as well [23,24], any of the other immune related genes in the MHC region could also be contributing to changes in cholesterol flux.