The case-only gene-gene interaction analysis, defined as deviation from the multiplicative model for the joint effects of the two genotypes, was performed using a regression model as a score test for association between genotypes in case subjects21. Affected sib pairs were not tested, as they are not independent. The HLA class II loci were grouped according to their genotypes using a risk-based method, rpart (S.N., J.M.M.H. and J.A.T., unpublished data; see URL below).
