Early genetic studies relied on positional cloning in combination with linkage analysis leading to detection of genes associated with asthma expressed in airway epithelium including A disintegrin and metalloprotease 33 (ADAM33), GPRA, protocadherin-1 (PCDH1), Serine protease inhibitor Kazal type-5 (SPINK5), IL-1 receptor associated kinase-M (IRAKM), Dipeptidyl-peptidase 10 (DPP10) and HLA-G genes (Table 1) [17–31]. Here, PCDH1 is linked to asthma.