Company: BLLN
Filing Date: 2025-10-07
Form Type: S-1
Source: 0001193125-25-233697
Chunk: 229

Company: BillionToOne, Inc.
Filing Date: 2025-10-07
Form: S-1
Chunk 229
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1 mutation at a very low VAF in a metastatic breast cancer patient after signs of progression using Northstar Response

A patient was diagnosed with HR positive/HER2 negative metastatic breast
cancer and was heavily pretreated with anastrozole/ribociclib. During the patient’s routine follow-up three years later, the patient began Northstar Response monitoring, which indicated low baseline
tumor burden (TMS=63). Longitudinal monitoring revealed progressively increasing TMS scores with a 390-fold increase found five months afterwards (TMS=67,000).

Subsequent comprehensive profiling with Northstar Select revealed an ESR1 S463P mutation at a VAF of 0.09%, significantly below any competing therapy selection
test’s limit of detection, confirming the emergence of a resistance mutation to aromatase inhibitor therapy, a common challenge in treating HR-positive metastatic breast cancer. ESR1 S463P
mutations have an FDA-approved therapy, elacestrant, in this indication and provided this patient with an opportunity for treatment.

This case highlights how utilizing both Northstar Select and Response can help physicians first timely detect disease progression and identify more actionable mutations
at greater sensitivity, leading to earlier interventions and better treatment options for patients.

Longitudinal Tumor Methylation ScoreTM (TMSTM) 100,000 10,000 1,000 100 25 170 63 Day 0 Treatment anastrozole + ribociclib 67,000 139 286 PROGRESSIVE DISEASE Key Insights from Northstar Baseline TMS Increase: ~3X Profiling Northstar Select GNAS R201H (0.08%) TMS Increase: ~390X Profiling Northstar Select CDK4 amp (8.86 copies) ESR1 S463P (0.09%) ESR1 L536P (0.76%) Treatment Implication Elacestrant (FDA approved in indication)

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Northstar patient case study: Synergistic use of Northstar Select and Response demonstrated true precision oncology and personalized medicine

A patient with metastatic colorectal cancer, awaiting first-line treatment with pembrolizumab monotherapy, presented with
a baseline TMS of 210,000, indicating a very high tumor burden. After the first cycle of treatment, initial molecular response was observed and TMS decreased by over 50%, while therapy profiling with Northstar Select identified a druggable
BRAF V600E mutation