Disintegrin and metalloproteinase domain-containing protein 33 (ADAM 33) is a single-pass type I membrane protein expressed in all tissues, except liver, with high expression in placenta, lung, spleen and veins. The conserved cysteine present in the cysteine-switch motif binds the catalytic zinc ion, thus inhibiting ADAM 33. The dissociation of the cysteine from the zinc ion upon the activation-peptide release activates ADAM 33. Genetic variations in ADAM33 are associated with susceptibility to asthma, the most common chronic disease affecting children and young adults. It is a complex genetic disorder with a heterogeneous phenotype, largely attributed to the interactions among many genes and between these genes and the environment. It is characterized by recurrent attacks of paroxysmal dyspnea, with weezing due to spasmodic contraction of the bronchi.