A modified version of the GRCh38 human reference genome that replaces the region chr21:6427259-6580181 with "N" characters. This creates a coordinate-compatible build that removes a problematic duplication region This duplicated sequence in the original reference causes mismapping or multimapping of reads to a locus that includes the genes CBS, U2AF1, FRGCA, and CRYAA and hinders variant detection in those genes. More details are available at https://www.ncbi.nlm.nih.gov/grc/human/issues/HG-2544 This reference is useful in applications where detection of U2AF1 mutations is critical, including studies of hematological cancers, or studies of spliceosome dysfunction. A publication with more details is forthcoming.