Inclusion Criteria:

* ≥18 years of age
* Willing and able to provide informed consent
* Histologically- or cytologically-confirmed metastatic or unresectable, recurrent HNSCC unsuitable for local therapies
* Intended for first line anti-PD1 or PDL1 monotherapy or combination therapy
* Must submit tumor tissue sample representative of current disease per laboratory manual

Exclusion Criteria:

* Non-squamous histologies (eg, nasopharynx or salivary gland)
* Relapse or recurrence within 6 months of first line chemotherapy and / or chemoradiotherapy
* Tumors that are PD-L1 negative (CPS \<1)
* Clinical evidence of an active second invasive malignancy within \<2 years of enrollment with the exception of stable prostate cancer on watchful waiting, in situ cervical cancer, in situ breast carcinoma or localized non-melanoma skin cancers
* Unable to comply with study procedures (i.e., not willing or able to have additional blood samples collected)

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 08/27/2025. Questions regarding updates should be directed to the study team contact.