CHD7 and Kallmann syndrome: Rare sequence variants (defined as genetic variants with a minor allele frequency lower than 1 % in control populations) in FGFR1 (10 %) and CHD7 (6 %) are the most common autosomal causes of Kallmann syndrome, whereas another causative gene, KAL1, has been estimated to have a prevalence of 5–10 % in affected males (X-linked recessive) (Layman 2013).