The possible effects of mutations on stability and function of a protein can only be understood in the context of protein 3D structure. The MUTATIONEXPLORER webserver maps variants onto protein structures and allows users to study variation by inputting sequence changes. As the user enters variants, the 3D model evolves, and estimated changes in energy are highlighted. In addition to a basic per-residue input format, MUTATIONEXPLORER can also upload an entire replacement sequence. Previously the purview of desktop applications, such an upload can back-mutate PDB structures to wildtype sequence in a single step. Structures are flexibly colorable, not only by energetic differences, but also by hydrophobicity, or sequence conservation, or other biochemical profiling. Another mutation source can be human single nucelotide polymorphisms (SNPs), genomic coordinates input in VCF format. MUTATIONEXPLORER strives for efficiency in user experience. For example, 45,000 PDB depositions for instant retrieval and initial display have been prepared. All modeling steps are performed by Rosetta.