To determine the starting point of the conversion event, we sequenced the 13kbp amplicon using a MinION nanopore sequencer and assessed known differences between OTOA and OTOAP1 reference genomic sequences (Fig. 2). The first OTOAP1 specific nucleotide was found approximately 350 bp upstream of OTOA exon 21. All expected downstream sequence differences corresponded to OTOAP1 sequences, at the hemizygous state, up to the end of the amplicon at exon 23. This fine mapping analysis allowed us to map the start of the conversion to a 334bp window, and to estimate its length to at least 9kbp. We reported the conversion with the following HGVS nomenclature: NC_000016.10:g.(21730155_21730489)_(21739516_?)con(22546282_22546616)_(22555638_?). Finally, we designed a 3kb PCR product around the mapped conversion start window with primers #1 and #7 (Fig. 1B, Supplementary Data) and confirmed the presence of the converted allele in the proband and her father, the conversion being absent in the mother (Fig. 1D).