Abstract We have previously described the first homozygous cases of Hb Knossos in an Algerian family. The Hb A 2 was completely absent, ascertaining the presence of a δ°-thalassemia determinant in cis of the β Knossoss gene. Here, we investigate the affected δ-globin gene. The complete DNA sequence of the gene and its 5′ and 3′ flanking regions was determined. Only two nucleotide changes were recorded: a C → T substitution at −199 and an AT insertion at −448 upstream from the cap site. To examine the involvement of these changes in gene function, the δ-gene was subcloned in an expression vector and introduced into COS cells. Analysis of RNA derived from these cells, using an S 1 protection assay and dot-blot hybridization, revealed qualitatively and quantitatively normal transcription. The loss of δ-globin gene activity in vivo may be due to the alteration of a tissue-specific control.