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Prevention of β thalassaemia implies knowledge of the molecular spectrum occurring in the population at risk. This knowledge is necessary, especially when a prevention protocol is applied to a multiethnic population. For this purpose, we carried out molecular analysis of 431 β thalassaemia subjects belonging to tribal (aboriginal) and non-tribal communities of Orissa, a part of peninsular India and found six types of mutation (four previously unreported and two reported). Molecular analysis of β gene mutation showed that out of 431 β thalassaemia cases (265 β thalassaemia traits, 64 β thalassaemia major, 47 haemoglobin E-β thalassaemia, 55 haemoglobin S-β thalassaemia cases), 71% of cases (n=306) showed the IVS I-5(G→C) mutation, 12% of cases (n=52) showed FS 41/42(-CTTT), 7% of cases (n=30) showed CD 15(G→A), 4·8% of cases (n=21) showed CD 30 (G→C), 3% of cases (n=13) showed FS 8/9 (+G), and 2% of cases (n=9) showed IVSI-1(G→T). The tribal populations possess only the IVS I-5(G→C) mutation whereas the non-tribal groups possess the FS 41/42(-CTTT), FS 8/9 (+G), IVS I-1(G→T), CD30(G→C) and IVS I-5(G→C) mutations. Among the non-tribal communities, Muslims did not have the IVS I-1 (G→T) mutation. Clinically, anaemia was mild to moderate in β thalassaemia trait and was found to be associated with the majority of abnormalities such as pyrexial episodes, fatigue, headache, lethargy and pallor. However, there were no differences in the incidence of clinical abnormalities between tribal and non-tribal populations and also among the different molecular variants of β gene. This is the first report from Orissa on the prevalence of different molecular variants of β thalassaemia. The clinical presentation of β thalassaemia trait cases and their variation from other population have been discussed with reference to the different genetic variants. [ABSTRACT FROM AUTHOR]