CryoFlex characterizes structural motion between conformations directly from cryo-EM density maps

Heterogeneous cryo-EM reconstruction methods can resolve multiple conformational states, but understanding their functional implications often requires determining which regions move between states and quantifying their displacement magnitudes. Comparing atomic models can quantify motion, but flexible regions are often incompletely modeled. Direct map comparison is limited by variable map quality and ambiguous correspondence between displaced density features. Here, we present CryoFlex, a method that estimates the direction and magnitude of structural motion directly from reconstructed density maps of selected states. The resulting motion analysis supports functional interpretation and guides downstream structure processing. CryoFlex localized and quantified structural motion across diverse map pairs, including those with incomplete atomic-model coverage or weak local density. Against C; displacements, CryoFlex achieved an endpoint error of about 1 angstrom. In separate benchmarks, it outperformed registration baselines on clean and noisy inputs. Thus, CryoFlex complements heterogeneous reconstruction workflows with direct quantification of structural motion between reconstructed states.