Collection of CSF Samples From Participants With Metastatic Triple Negative Breast Cancer (TNBC) and HER2+ Breast Cancer With no Prior History Nor Active Radiographically Detectable Brain Metastases

Background: Breast cancer is the most common cancer among women. It can often spread to the liver, lungs, bones, or brain. Breast cancer that spreads to the brain is often fatal. Researchers want to know if tumor DNA found in spinal fluid, blood, or tumor tissue can help predict when the cancer will spread to the brain. They want to collect these fluid and tissue samples for research. Objective: To collect spinal fluid and other samples from people with breast cancer that has spread to other parts of the body. Eligibility: People aged 18 years and older with HER2-positive or triple negative breast cancer. The cancer must have spread to other parts of the body but not to the brain. Design: Participants will be screened. They will have blood tests to assess kidney function. They will have an imaging scan of the brain. Participants will come to the NIH clinic to have their samples collected: * Spinal fluid. A thin needle will be inserted into the lower back to draw out a sample of fluid from the space around the spinal cord. A physical exam and blood tests will be done to make sure it is safe for participants to have this procedure. * Blood. * Saliva or cheek swabs. They will rub a cotton swab inside of their mouth. * Tumor samples. If participants have had samples of tumor tissue (biopsies) collected in the past, leftover tissue may be used for this study. Participants will be contacted for follow-up every 6 months for 3 years. They may return once a year to provide further samples.

* INCLUSION CRITERIA: * Pathology documentation of histologically confirmed HER2+ BC or TNBC with a history of metastatic disease. * Participants must be able to undergo lumbar puncture (LP) and brain MRI. * Women age \>= 18 years * Adequate organ function as defined below: * Creatinine \=40 mL/min/1.73 m2 for individuals with creatinine levels above institutional ULN (using either Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] equation) \- Participants must be able to understand and willing to sign a written informed consent document. EXCLUSION CRITERIA: * Prior history or current MRI-detected brain metastasis or leptomeningeal disease * Previous history of any invasive malignancies, except for surgically resected local cutaneous malignancies. * Pregnancy