L1 associated breakpoints and sequences. (A) Six L1 sequences in the 210-kb region are aligned to the active LINE-1 (L1.2) prototype sequence (46). These L1 elements showed >80% sequence homology to the prototype over 1,000 bp. Note that all L1s were truncated, retaining only a 3′ portion of the sequence, and most breakpoints were located at the 5′ end of the retained sequence. EN, endonuclease; RT, reverse transcriptase; C, cysteine-rich region. (B) Rearrangements and sequence of breakpoints in MB436 and LS180; note sequence homology to the L1 prototype.