Mercaptopurine for the Treatment of Hereditary Leiomyomatosis and Renal Cell Carcinoma (HLRCC), Uterine/Cutaneous Leiomyomas, and Kidney Cancer

Inclusion Criteria: * Age ≥ 18 * Disease-causing, germline FH mutation including variants considered either: * a) Pathogenic/likely pathogenic OR * b) variants of unknown significance (VUS) with immunohistochemical staining showing loss of FH or high 2-SC expression in tumor tissue * Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤ 1 * Thiopurine S-methyltransferase (TPMT) and NUDT15 homozygous, wild-type genotype * TPMT\*1/TPMT\*1 and NUDT15\*1/ NUDT15\*1 * Calculated creatinine clearance ≥ 30 milliliters per minute (mL/min) per the Cockcroft and Gault formula OR serum creatinine \< 1.5 x upper limit of normal (ULN) * Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \< 3 x ULN (\< 5 x ULN if liver metastases are present) * Total bilirubin \< 1.5 x ULN (except subjects with Gilbert Syndrome, who can have total bilirubin up to 3.0 mg/dL) * Albumin ≥ 2.2 mg/dL * White blood cells (WBC) \> 2,000/mm\^3 * Hemoglobin (Hb) ≥ 9 * Neutrophils \> 1,500/mm\^3 * Platelets \> 100,000/mm\^3 * Inclusion into ≥ 1 of the following symptomatic, disease states listed below: * Inclusion allows entry into that cohort for efficacy assessments. Patients can be in ≥ 1 cohort if they have more than one disease manifestation fitting the below criteria. If a patient fits inclusion/exclusion into one cohort and has disease manifestations that are excluded from another cohort, they can still participate in the trial but will not be assessed for efficacy fo…