The regulatory networks governing development and differentiation of hematopoietic cells are incompletely understood. Members of the Schlafen protein family have been implicated in the regulation of cell growth and T cell development. Four new members, Schlafen 5, Schlafen 8, Schlafen 9 and Schlafen 10, which belong to a distinct subgroup within this gene family have been identified and chromosomally mapped. The characteristic feature of these proteins is the presence of sequence motifs identifying them as distinct members of the superfamily I of DNA/RNA helicases.