In this paper we present the ongoing development and extension work on BioTop--a top-domain ontology for linking biomedical domain ontologies. We start by making the case for the application of a common ontology to interface independent biomedical domain ontologies by introducing a set of more general classes. Then we briefly depict the relation of BioTop to the GENIA ontology as starting point of its initial developement. Afterwards we propose our distinction of ontologies into top, top-domain and domain ones and describe our approach to the integration of the top ontology BFO into BioTop. Then we present our plans to join the OBO and OBO Foundry repository of ontologies and list its admission principles in relation to our ontology. Some actual BioTop interface classes are shown subsequently. We conclude by detailing on some planned BioTop usages in the area of BioNLP and cancer research and show some further intended improvements.