SMA is an autosomal-recessive neurodegenerative disease with variable expression, caused by deletions, gene conversions or mutations in the SMN1 gene.29, 30 With an incidence of around 1 in 10 000 live births, it is the second most common autosomal-recessive disorder after cystic fibrosis in the northern European population.31 SMN1 maps to a complex region of chromosome 5q13.1, where an element of around 500 kb containing four genes (SMN, NAIP, SERF and GTFH2) is present in inverted duplicate copies. The gene discussed is SMN1; the disease is cystic fibrosis.