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Application-layer overlay networks have recently emerged as a promising solution for live media multicast on the Internet. A tree is probably the most natural structure for a multicast overlay, but is vulnerable in the presence of dynamic end-hosts. Data-driven approaches form a mesh out of overlay nodes to exchange data, which greatly enhances the resilience. It however suffers from an efficiency-latency tradeoff, given that the data have to be pulled from mesh neighbors with periodical notifications. In this paper, we suggest a novel hybrid tree/mesh design that leverages both overlays. The key idea is to identify a set of stable nodes to construct a tree-based backbone, called treebone, with most of the data being pushed over this backbone. These stable nodes, together with others, are further organized through an auxiliary mesh overlay, which facilitates the treebone to accommodate node dynamics and fully exploit the available bandwidth between overlay nodes. This hybrid design, referred to as mTreebone, is braced by our real trace studies, which show strong evidence that the performance of an overlay closely depends on a small set of backbone nodes. It however poses a series of unique and critical design challenges, in particular, the identification of stable nodes and seamless data delivery using both push and pull methods. In this paper, we present optimized solutions to these problems, which reconcile the two overlays under a coherent framework with controlled overhead. We evaluate mTreebone through both simulations and PlanetLab experiments. The results demonstrate the superior efficiency and robustness of this hybrid solution.