Squirrel monkey retrovirus (SMRV) is a xenotropic endogenous virus of the squirrel monkey. The long terminal repeat of the SMRV contains four direct repeats (43 or 42 bp) with different degrees of perfection in the U3 region. Herein, we found two human nuclear proteins (SMBP1 and SMBP2) which bind specifically to the direct repeats with different affinities. The binding proteins recognize overlapped sequences in the 5' region of the repeats. SMBP1 and 2 are monomeric proteins of 35 and 17 kDa, respectively.
