Partial replication is a way to increase the scalability of replicated systems: updates only need to be applied to a subset of the system's sites, thus allowing replicas to handle independent parts of the workload in parallel. In this paper, we propose P-Store, a partially replicated key-value store for wide area networks. In P-Store, each transaction T optimistically executes on one or more sites and is then certified to guarantee serializability of the execution. The certification protocol is genuine, it only involves sites that replicate data items read or written by T, and incorporates a mechanism to minimize a convoy effect. P-Store makes a thrifty use of an atomic multicast service to guarantee correctness: no messages need to be multicast during T's execution and a single message is multicast to certify T. In case T is global, that is, T's execution is distributed at different geographical locations, an extra vote phase is required. Our approach may offer better scalability than previously proposed solutions that either require multiple atomic multicast messages to execute T or are non-genuine. Experimental evaluations reveal that the convoy effect plays an important role even when one percent of the transactions are global. We also compare the scalability of our approach to a fully replicated solution when the proportion of global transactions and the number of sites vary.