Interaction of proteins HMG1 and HMG2 with Form X. (A) It was previously observed that a double-stranded 120 bp DNA fragment containing a 60 bp tract of the repetitive sequence poly(CA)· poly(TG), when end-labelled and used in a gel retardation experiment with a nuclear extract of cultured monkey cells (CV1 line) in the presence of high amounts of E. coli competitor DNA, could give rise to two retarded bands, C1 and C2, corresponding to specific DNA-protein complexes . (B) The proteins responsible for the formation of retarded bands C1 and C2 were purified and identified as proteins HMG1 and HMG2 . (C) The DNA contained in retarded bands C1 and C2 was purified: on a polyacrylamide gel it migrated more slowly than the regular double-stranded fragment and showed a series of bands, initially named "Form X", which reformed complexes C1 and C2 by interaction with HMG1/2, and which have been identified  as DNA loops maintained at their base by a semicatenated DNA junction. A highly schematic representation of the structure of Form X is shown, showing the junction in which two DNA duplexes cross with one of the strands of one duplex passing between the two strands of the other duplex, and reciprocally.