Developing for mobile and multimodal platforms is more important now than ever, as smartphones and tablets proliferate and mobile device orchestras become commonplace. We detail NexusUI, a JavaScript framework that enables rapid prototyping and development of expressive multitouch electronic instrument interfaces within a web browser. Extensions of this project assist in easily creating dynamic user interfaces. NexusUI contains several novel encapsulations of creative interface objects, each accessible with one line of code. NexusUp assists in one-button duplication of Max interfaces into mobile-friendly web pages that transmit to Max automatically via Open Sound Control. NexusDrop enables drag-and-drop interface building and saves interfaces to a central Nexus database. Finally, we provide an overview of several projects made with NexusUI, including mobile instruments, art installations, sound diffusion tools, and iOS games, and describe Nexus' possibilities as an architecture for our future Mobile App Orchestra.