Since a few years it has become clear that it is possible to apply topological methods to characterize the structure of proteins. For instance, knots and “lasso” structures were found in several proteins. Here we focus on the linking number quantifying the degree of entanglement between two loops, which may be easily generalized to estimate the entanglement between two open curves such as protein backbones. With this scheme we find a high degree of interwinding between proteins forming domain-swapped dimers. In the same way, one may estimate the maximum degree of entanglement within subchains of a single protein backbone to find that it strongly anti-correlates with that protein folding rate. This introduces a novel characterization of proteins that complements other methods extracting information from their tertiary structure.