CARASIL is caused by mutation of the HTRA1 gene which encodes the HtrA serine peptidase 1 protein (HTRA1). HTRA1 is located on chromosome 10 and encodes an enzyme that regulates signaling by the TGF-β family of proteins. TGF-β protein family plays an important role in cellular functions, especially in angiogenesis. Individuals with CARASIL have mutations in HTRA1 which leads to a reduced amount of HTRA1 protein or no HTRA1 protein at all. The mutant proteins are unable to suppress TGF-β activity. Increase in TGF-β1 activity has been seen in the tunica media of affected small arteries. CARASIL is an<unk><unk>  disease, meaning that both parents must be a carrier for the allele in order for the disease to be passed on to the child. As with other<unk><unk>  diseases, the likelihood of receiving a<unk>  allele from both parents increases if the parents are closely related to each other (consanguineous). This trend has been observed for CARASIL.
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