An MCNPX developer works on the Monte Carlo N-Particle eXtended software, a comprehensive toolkit for simulating nuclear processes. This includes designing, coding, and testing the software to ensure it accurately simulates particle transport and interactions, from high-energy accelerators to isotopic production. They also work on improving the software's performance, expanding its capabilities, and integrating it with other systems. Moreover, they need to understand complex nuclear physics concepts, algorithms, and computational methods. They also provide technical support, training, and documentation for users, and collaborate with scientists and engineers to use MCNPX for various research and industrial applications.