The partners of the ProteomeXchange consortium have been developing an API for the communication of proteomics data called ProXI, formally the ProteomeXchange eXpression Interface. The API enables the programmatic access to information about datasets, proteins, peptides, peptidoforms, peptide-spectrum matches (PSMs), and spectra via a consistent interface for all ProteomeXchange partners as well as an aggregator at ProteomeCentral. ProXI remains a work in progress, with several of the endpoints designed and at least partially implemented at several partners. Dedicated funding is likely required to complete it. PROXI is the mechanism that drives the multi-repository USI lookup and display mechanism at ProteomeCentral described above. ProXI uses the OpenAPI 3.0 platform for designing endpoints with a JSON communication format. The dataset endpoint is modeled after the PX XML schema, and the spectrum endpoint is modeled using the mzSpecLib schema described above.