(16p13). TSC-LAM occurs in women who have germline mutations in either the TSC1 or the TSC2 gene. Sporadic LAM is primarily associated with somatic TSC2 gene mutations. Germline and somatic mutations in LAM include many types of mutations spread across the genes, with no clear “hot spots,” including missense changes, in-frame deletions and nonsense mutations. Because of the large size of the genes (together they have more than 60 exons) and because mutations can be located virtually anywhere within the genes, mutation detection is often challenging. On a cellular basis, LAM cells carry bi-allelic inactivation of the TSC2 genes, consistent with the “two-hit” tumor suppressor gene model. The second hit event in LAM cells is often loss of the chromosomal region containing the wild-type copy of the TSC2 gene; this is referred to as loss of heterozygosity or LOH. LOH can be detected in microdissected LAM cells, in angiomyolipomas and lymph nodes from women with LAM, and in circulating LAM cells (cells in blood and urine). Angiomyolipomas and pulmonary LAM cells from women with the sporadic form of LAM carry identical mutations in TSC2. This, together with the fact that recurrent LAM after lung transplantation carries the same TSC2 mutations as the original LAM, has led to the "benign metastasis" hypothesis that LAM cells can migrate or metastasize from one site to another. Pathophysiology A variable percentage of cells within the LAM lesion contain mutational inactivation of the tuberous sclerosis complex (TSC1 or TSC2) tumor suppressor genes. TSC1 mutations cause a less severe clinical phenotype than TSC2 mutations. The discovery of TSC1/2 gene function as negative regulator of the mammalian target of rapamycin complex 1 (mTORC1) led to successful use of rapamycin analog sirolimus in clinical trials and FDA approval of sirolimus for treatment of LAM. TSC1 and TSC2 form a tumor suppressor complex that regulates mammalian target of rapamycin (mTOR) signaling complex by directly controlling the activity of the small GTPase Rheb via the GTPase activating protein (GAP) domain of TSC2. Rheb binds to Raptor and controls the activity of mTOR complex 1 (mTORC1) that directly phosphorylates p70 S6 kinase (S6K1) and 4E-BP1. mTOR forms two physically and functionally distinct multiprotein complexes: the rapamycin-sensitive mTORC1 and the rapamycin-insensitive mTORC2. MTORC1 consists of five proteins including Raptor that positively regulate mTOR activity. MTORC2 consists of six proteins including mTOR and Rictor, which defines the activation level of mTORC2 and modulates