blm arises from a mutation in the top2a gene. A) Mapping of blm reveals that the gene is located on chromosome 12. B) top2a was selected as a candidate gene and sequencing of mutant cDNA shows a point mutation (A→T) in Lys residue 335 resulting in a premature stop codon. C) A schematic of wildtype Top2a protein and the truncated proteins in hi3635, blm and can4 mutants D) Complementation assay performed to confirm that blm is top2a mutation. blm heterozygotes were crossed to hi3635 carriers and the resulting hybrid offspring present with blm phenotype.