The NssBF element has been characterized as a 26 nt sequence in the long terminal repeat of Drosophila melanogaster retrotransposon 1731. This sequence has been shown to be implicated in transcriptional repression of the 1731 promoter. We here report the cloning of a cDNA encoding a nuclear DNA binding protein named p11 that binds specifically to the NssBF element. P11 is a 98 amino acid polypeptide. It exhibits similarities with the mouse p9 single-stranded DNA binding protein, raising the possibility of a very general family of protein factors. Co-transfection experiments in human U937 cells showed repression of the 1731 promoter by overexpression of p11.