New Data Demonstrating Accuracy of DecisionDx® DiffDx™-Melanoma Presented at ASDP 58th Annual Meeting

Presentation highlights data from second independent study validating DecisionDx® DiffDx™-Melanoma as an ancillary melanoma diagnostic tool for difficult-to-diagnose melanocytic lesions

Castle Biosciences, Inc. (Nasdaq: CSTL), a company applying innovative diagnostics to inform disease management decisions and improve patient outcomes, today announced new data further validating the performance of the Company’s DecisionDx® DiffDx™-Melanoma test as a highly accurate, ancillary tool to aid in the diagnosis of melanocytic lesions with ambiguous histopathology. The data was shared in a poster presentation at The American Society of Dermatopathology (ASDP) 58^th Annual Meeting, held virtually Oct. 20-24.

DiffDx-Melanoma is part of Castle’s Comprehensive Diagnostic Offering (CDO), leveraging the strengths of both myPath® Melanoma and DiffDx-Melanoma. These two gene expression profile (GEP) tests were designed to provide a highly accurate, objective result to aid dermatopathologists and dermatologists in characterizing difficult-to-diagnose melanocytic lesions.

The poster, titled “Performance of the 35 Gene Expression Profile (GEP) Test for Use as an Adjunctive Melanoma Diagnostic Tool in a New Independent Validation Cohort,” describes the performance of DiffDx-Melanoma, independent of myPath Melanoma and also as part of the clinical CDO workflow. The poster can be viewed here.

“In clinical practice, difficult-to-diagnose melanocytic lesions often present a challenge for dermatopathologists, particularly when there is hesitation in the diagnosis of melanoma,” said Gregory A. Hosler, M.D., Ph.D., dermatopathologist at ProPath in Dallas. “In this study, DiffDx-Melanoma demonstrated its ability to provide dermatopathologists additional information to help guide and potentially increase confidence in a diagnosis, if any uncertainty exists, to help clinicians provide their patients with the most appropriate care.”

Study background and findings:

• The Institutional Review Board (IRB)-approved study reviewed 644 melanocytic lesions and associated de-identified clinical data from patients ≥ 18 years of age.
• Lesions were independently reviewed by at least two dermatopathologists for diagnostic adjudication and included in the study if they received at least two out of three reviews that were diagnostically concordant.
• Accuracy metrics demonstrated highly accurate performance of the DiffDx-Melanoma test (96% sensitivity and 92.2% specificity; intermediate result 2.3%) across lesion subtypes reported (excluding spitzoid lesions), including those with subtype discordance.
• Separately, the clinical Comprehensive Diagnostic Workflow leverages myPath Melanoma with DiffDx-Melanoma. Clinical order data from June 3-Aug. 31, 2021, showed that this workflow was able to increase the reporting of actionable test results of either benign or malignant from 78% for myPath Melanoma alone to 99% when used in conjunction with DiffDx-Melanoma.
• Overall, the study data demonstrated that DiffDx-Melanoma was an accurate, objective ancillary tool that provided information to aid in the diagnosis of melanocytic lesions of uncertain malignant potential, thus enhancing diagnostic confidence and the delivery of clinically, actionable results to healthcare providers.

About Castle Biosciences’ Comprehensive Diagnostic Offering for Difficult-to-Diagnose Melanocytic Lesions

Castle Biosciences’ comprehensive diagnostic offering leverages the strengths of myPath® Melanoma and DecisionDx® DiffDx™-Melanoma. These gene expression profile tests are designed to provide a highly accurate, objective result to aid dermatopathologists and dermatologists in characterizing difficult-to-diagnose melanocytic lesions. Of the approximately 2 million suspicious pigmented lesions biopsied annually in the U.S., Castle estimates that approximately 300,000 of those cannot be confidently classified as either benign or malignant through traditional histopathology methods. For these cases, the treatment plan can also be uncertain. Obtaining highly accurate, objective ancillary testing can mean the difference between a path of overtreatment or the risk of undertreatment. Interpreted in the context of other clinical, laboratory and histopathologic information, myPath Melanoma and DecisionDx DiffDx-Melanoma are designed to reduce uncertainty and provide confidence for dermatopathologists and help dermatologists deliver more informed patient management plans.

More information about the test and disease can be found at www.CastleTestInfo.com.

About Castle Biosciences

Castle Biosciences (Nasdaq: CSTL) is a commercial-stage diagnostics company focused on providing physicians and their patients with personalized, clinically actionable genomic information to make more accurate treatment decisions. The Company currently offers tests for patients with cutaneous melanoma (DecisionDx®-Melanoma, DecisionDx® -CMSeq), cutaneous squamous cell carcinoma (DecisionDx®-SCC), suspicious pigmented lesions (myPath® Melanoma and DecisionDx® DiffDx™-Melanoma) and uveal melanoma (DecisionDx®-UM, DecisionDx®-PRAME and DecisionDx®-UMSeq). For more information about Castle’s gene expression profile tests, visit www.CastleTestInfo.com.

Castle also has active research and development programs for tests in other dermatologic diseases with high clinical need, including its test in development to predict systemic therapy response in patients with moderate to severe psoriasis, atopic dermatitis and related conditions. Castle Biosciences is based in Friendswood, Texas (Houston), and has laboratory operations in Phoenix.

For more information, visit www.CastleBiosciences.com.

DecisionDx-Melanoma, DecisionDx-CMSeq, DecisionDx-SCC, myPath Melanoma, DecisionDx DiffDx-Melanoma, DecisionDx-UM, DecisionDx-PRAME and DecisionDx-UMSeq are trademarks of Castle Biosciences, Inc.

Forward-Looking Statements

The information in this press release contains forward-looking statements and information within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, which are subject to the “safe harbor” created by those sections. These forward-looking statements include, but are not limited to, statements concerning Castle’s Comprehensive Diagnostic Offering’s ability to provide a highly accurate, objective result to aid dermatopathologists and dermatologists in characterizing difficult-to-diagnose melanocytic lesions, enhance diagnostic confidence and deliver clinically actionable results to healthcare providers. The words “anticipates,” “believes,” “estimates,” “expects,” “intends,” “may,” “plans,” “projects,” “will,” “would” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. We may not actually achieve the plans, intentions or expectations disclosed in our forward-looking statements, and you should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward-looking statements involve risks and uncertainties that could cause our actual results to differ materially from those in the forward-looking statements, including, without limitation, the effects of the COVID-19 pandemic on our business and our efforts to address its impact on our business, subsequent study results and findings that contradict earlier study results and findings, Castle’s Comprehensive Diagnostic Offering’s ability to provide the aforementioned benefits to patients and the risks set forth in our Quarterly Report on Form 10-Q for the quarter ended June 30, 2021, and in our other filings with the SEC. The forward-looking statements are applicable only as of the date on which they are made, and we do not assume any obligation to update any forward-looking statements, except as may be required by law.

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