Background: The automated transfer of discrete molecular data from pathology labs into the electronic health record (EHR) is rapidly becoming commonplace and is designed to facilitate clinical decision making. Simple genomic changes are represented in existing EHR data transfer standards, but current data standards lack clinical fields that correspond to treatment insights such as associated medications and potential clinical trials. To address this, the mCODE effort defines a consensus oncology data standard that facilitates lightweight transmission of data from patients with cancer. mCODE’s support of genomic data leverages efforts by the HL7 Clinical Genomics Working Group (HL7 CGWG) focusing on oncology. Here, we describe pilot efforts involving both a pathology partner and academic medical centers to extend mCODE FHIR (Fast Healthcare Interoperability Resources) implementation guide (IG) support of genomic data for data interchange and clinical decision support use cases. Methods: From 2021-2022, validation of the mCODE FHIR IG genomic data elements was performed in CodeX, a FHIR accelerator program. These elements encompassed: lab facility, test order information, pathologist, patient demographics, specimen, molecular studies and therapeutic implications. Genomic FHIR data elements were selected for their conciseness and clinical relevance for inclusion in the mCODE FHIR IG under the supervision of two oncology informaticians. As part of the mCODE FHIR IG pilot, synthetic solid tumor molecular patient reports were generated by Tempus Labs. These synthetic molecular reports were transformed into mCODE-conformant FHIR files for transfer. The mCODE IG can be found at http://hl7.org/fhir/us/mcode/. Results: Over 50 Tempus data elements were analyzed and found to be readily mapped to mCODE genomics elements. Two categories of data elements required additional transformation: 1. Data elements requiring additional genomic database annotations; 2. FHIR syntax constraints like the concatenation of medications in the same drug class. The mCODE file was transmitted to Vanderbilt and the Children's Hospital LA from Tempus for decoding and downstream uses. This effort further informed the HL7 CGWG of needed IG clarifications for compatibility with other standardization efforts and genomic use cases. Conclusions: mCODE’s support of genomic data elements provides a lightweight genomics interchange standard compatible with an existing pathology lab report. Mapping from existing molecular pathology reports to mCODE required nominal alterations. Furthermore, mCODE’s alignment with HL7 genomics efforts ensure FHIR-based compatibility in other clinical domains and with developing international standards. Continued pathology lab and academic medical center collaboration is essential for mCODE refinement and adoption.