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y parents display transfusion-dependent hypochromic anemia. A nonsense STEAP3/TSAP6 was identified in the siblings at the heterozygous state. This mutation was inherited from their father while no mutation was found in their mother. A large variability of expression was found between normal alleles in a control population, confirming a previous report that STEAP3/TSAPS6 is an expressed quantitative trait locus (e-QTL). Determination of the relative allele expression showed that the "normal" allele was expressed at a significantly higher level in the father than in the affected siblings relative to the shared mutated allele. The blood level of STEAP3/TSAP6 mRNA was severely reduced in the siblings, while both parents were in the lower range of normal controls. The STEAP3/TSAP6 protein was also reduced in lymphocytic cell lines from the patients. Collectively, our data support the hypothesis that STEAP3/TSAP6 deficiency leads to severe anemia in the affected siblings and results from the combination of a mutated allele inherited from their father and a weakly expressed allele inherited from their mother.