Snomed ct is a large-scale medical ontology, which is developed using a variant of the inexpressive Description Logic EL. Description Logic reasoning can not only be used to compute subsumption relationships between Snomed concepts, but also to pinpoint the reason why a certain subsumption relationship holds by computing the axioms responsible for this relationship. This helps developers and users of Snomed ct to understand why a given subsumption relationship follows from the ontology, which can be seen as a first step toward removing unwanted subsumption relationships. In this paper, we describe a new method for axiom pinpointing in the Description Logic EL+ , which is based on the computation of so-called reachabilitybased modules. Our experiments on Snomed ct show that the sets of axioms explaining subsumption are usually quite small, and that our method is fast enough to compute such sets on demand.