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Introduction: Unstable hemoglobin disorders are characterized by a congenital, mostly familial chronic hemolytic anemia with episodes of severe hemolysis during febrile illnesses. Usually, isopropanol and heat stability tests lead to the diagnosis which is confirmed by protein and gene sequencing. Generation of the mutations is still a subject of controversy. Patient, materials and methods: We describe a 6-year-old Swiss child with congenital hemolytic anemia and a negative family history. Hemoglobin was studied by IEF, HPLC reverse phase chromatography, heat stability and isopropranol tests. DNA was sequenced in both coding and non-coding strands. Results: An unstable Hb was diagnosed on the basis of positive heat stability and isopropranol tests. The TTT→TTG mutation at codon 42 corresponding to a Phe→Leu substitution was found on DNA sequencing. Paternity was confirmed indicating that we are dealing with a new mutation. Conclusions: So far, three different mutations at codon 42 of the β-chain, and two at the corresponding position of the α-chain have been described, all leading to a hemolytic anemia. These mutations can either represent random phenomena occurring at an important location in the heme pocket, or may be secondary to the two highly homologous zones present in this region. These homologous zones may indicate a hot spot for point mutations created by abnormal crossing over or formation of loops, and an imperfect DNA repair process. [ABSTRACT FROM AUTHOR]