Model of Form X: a DNA loop with a semicatenated DNA junction. After denaturation, the reassociation of the strands of a DNA fragment containing the sequence poly (CA) · poly (TG) can occur with a shift in the repetitive sequence. In such a case, the reassociation is expected to pause when it reaches the sides of the repetitive region, allowing one of the single-stranded ends to insert into the fork formed by the two single strands at the opposite end. This process is facilitated by HMG1 or HMG2, and possibly also by the presence of complementary sequences left on both sides of the double-stranded region. The final result is a loop at the base of which two DNA duplexes cross, forming a knot in which one of the strands of one duplex passes between the two strands of the other duplex, and reciprocally, to form a semicatenated DNA junction.