The upper panel represents a typical Alu repeat consisting of left and right arms with the poly A linker. The A and B regions are sequences homologous to promoters of genes transcribed by RNA polymerase III. The arrow indicates the presumed 5′ breakpoint in intron 9 of the GPIIIa gene creating the I-J₂ mutation. The lower panel shows the sequences in proximity to the breakpoints in intron 9 and in exon 13. The GGTC identical sequence in intron 9 and exon 13 is in bold letters, with the putative breakpoints depicted. Direct/inverted repeats frequently observed close to breakpoints are underlined. CTG/CAG repeats suggested to be recognition sites for topoisomerase I are marked by bold lines above letters.