Abstract: The present invention relates, in one aspect, to a method for determining the severity of a disease attributed to at least one genetic mutation in one or more of the genes encoding haemoglobin polypeptide chains, comprising the steps of: (a) providing a sample from said subject; and (b) determining the presence of one or more diagnostic markers: (i) within a 127 kb segment on chromosome 2p15; (ii) within MYB and/or HBSIL and/or the intergenic region between MYB and HBSIL located on the 6q23 QTL interval; and/or (iii) within one of the chromosomal loci given in Table 14; wherein the presence of said marker(s) in said sample is indicative that the severity of said disease in said subject will be or is less severe in said subject in comparison to a subject that does not possess said marker(s).