Provides a literature-derived database of immune cell markers
formatted as Gene Matrix Transposed (GMT) files. Users can search immune
cell signatures, retrieve marker lists, export GMT files, create custom
marker sets, and score gene-by-cell expression matrices with dependency-free
rank-based or mean-expression methods. Cell subpopulations are distinguished
by their source PMIDs. For the core curation of the lung cell atlas,
see Travaglini et al. (2020)