[A case of 9p-syndrome identified chromosome 20 on chromosome 9p by M-FISH].

In this report, we describe one-year-old girl diagnosed with 9p-syndrome. Cytogenetic studies of this patient confirmed a karyotype of 46,XX,add(9) (p24) chromosome, but could not find the additional fragment on 9p22 in one allele. Fluorescence in situ hybridization (FISH) studies could not confirm the fragment in the patient using the LIS1 gene probe which mapped to 9p22. The more recently developed M-FISH method clearly showed that the additional fragment was 20p in this patient. These findings suggest that M-FISH analysis may be a useful method for identifying unknown additional and rearranged chromosomes.