In biology, the construction of plasmids is a routine technique, yet under-optimal, expensive and time-consuming. In this paper, we model the Plas-mid Cloning Problem (PCP) in constraint program-ing, in order to optimize the construction of plas-mids. Our technique uses a new propagator for the AtMostNVector constraint. This constraint allows the design of strategies for constructing multiple plasmids at the same time. Our approach recommends the smallest number of different cloning steps, while selecting the most efficient steps. It provides optimal strategies for real instances in gene therapy for retinal blinding diseases.