Castle Biosciences is a molecular diagnostics company that develops and sells proprietary genomic tests used by clinicians to guide treatment decisions for patients with skin cancers, Barrett's esophagus, and atopic dermatitis. Castle's tests are multi-analyte gene expression profile assays — they analyze gene expression from patient tissue samples and run results through a proprietary algorithm to generate a risk score or classification. Castle sells its tests through a direct U.S. sales force calling on dermatologists, gastroenterologists, and ophthalmologists. Castle's business model is simple: perform a diagnostic test, then bill Medicare or commercial insurers. Revenue is driven by test volume and reimbursement rates per test. Reimbursement is the critical variable — Medicare covers roughly 44% of revenue, and a single commercial payor covers about 16%. Castle's largest test is DecisionDx-Melanoma, which predicts metastasis and recurrence risk for invasive cutaneous melanoma patients, with an estimated ~28% penetration of its target market. TissueCypher, which predicts Barrett's esophagus patients' risk of progressing to esophageal cancer, is the fastest-growing test with roughly 5% penetration. Castle also offers tests for squamous cell carcinoma (currently facing a Medicare noncoverage headwind), uveal melanoma, and a newly launched atopic dermatitis treatment-selection test. Gross margins are in the mid-to-high 70s, with sales and marketing as the largest operating expense. Castle's growth strategy centers on deeper penetration of existing tests, new test development, and selective M&A targeting tests with established reimbursement that complement its existing portfolio.