Not all RA cases carry alleles that belong to the shared epitope. No more than 87% of cases are positive for SE. The SE-negative cases of RA could be explained through the hypothesis of exposure to noninherited SE positive maternal alleles during foetal development (Newton et al. 2004). Specifi cally, 30% of patients of RA from United Kingdom regions for example, do not carry allele DR4; meanwhile, only 15% are SE positive. There may be a number of mechanisms that explain this divergence. These mechanisms may include the process of microchimerism and inherited alleles from the foetus may give rise to post-partum arthritis in mothers, among others.