• PTEN hamartoma tumor syndrome is caused by heterozygous, loss of function variants in the PTEN gene. • Features include benign tumors (e.g., skin hamartomas, gastrointestinal polyps, thyroid nodules/goiter, fibrocystic breast disease, cerebellar dysplastic gangliocytoma), and increased risk for certain cancers (e.g., breast, thyroid, renal cell, endometrial, colorectal, and melanoma). • Expressivity is highly variable between and within families. The gene discussed is PTEN; the disease is Lhermitte-Duclos disease.