2. Availability of an HLA-identical sibling donor (MRD) or HLA-identical unrelated donor (MUD). Donor selection is based on molecular high resolution typing (4 digits) of class II alleles of the DRB1 and DQB1 gene loci and molecular (at least) low resolution typing (2 digits) of class I alleles (i.e., antigens) of the HLA- A, B, and C gene loci. In case, no class I and class II completely identical donor (10 out of 10 gene loci) can be identified, one antigen disparity (class I) and/or one allele disparity (class II) between patient and donor are acceptable. Conversely, disparity of two antigens (irrespective of the involved gene loci) cannot be accepted. These definitions for the required degree of histocompatibility apply to the selection of related as well as of unrelated donors.