This is the extdata/constant_regions/IMGT/ folder in the igblastr package.

This folder contains the C-regions (in FASTA format) obtained from IMGT for
human, mouse, and rabbit.
Note that these FASTA files were obtained programmatically by running the
following code in the folder on Dec 17, 2024:

    igblastr:::download_all_C_regions_from_IMGT()

Each of the human/, mouse/, and rabbit/ subfolder has a 'version' file that
contains the date of the download in YYYYMM format.

