Metachromatic leukodystrophy (MLD) is a genetic disease that affects the nervous system. MLD is caused by mutations in the ARSA gene located on chromosome 22. The ARSA gene, which is 3150 base pairs (bp) in length, includes 8 exons and is shown in Fig. 5.1. exon 3.15 kb Fig. 5.1 A genetic test using DNA sequencing is available to identify mutations associated with MLD in the ARSA gene. The sequencing method can only work if a DNA fragment size is less than 1000 bp. The test involves a number of different stages: • Genomic DNA is extracted from a blood sample. • Polymerase chain reaction (PCR) using 5 pairs of primers selects 5 different DNA fragments of the ARSA gene. • Gel electrophoresis is carried out on the DNA fragments. • The DNA fragments are sequenced and analysed for mutations. Table 5.1 shows the length of the DNA fragments and the exons present in each DNA fragment. Table 5.1 fragment number | DNA fragment length / bp | exons in DNA fragment 1 | 405 | 1 2 | 737 | 1-2 3 | 706 | 2-4 4 | 860 | 5-7 5 | 916 | 7-8