• Histologically confirmed small cell lung cancer diagnosis

• Diagnosis made with excisional or core needle biopsy specimen (fine needle aspirate may be permitted with approval from the PI)

• Subjects must submit fresh or archived FFPE tumor sample to the central laboratory

• ECOG performance status of 0-2 at time of enrollment

• Planned extensive stage first-line therapy of etoposide plus carboplatin plus PD-L1 inhibitor (atezolizumab or durvalumab)

• Extensive stage disease at time of diagnosis

• Willing and able to provide informed consent

• Radiotherapy is permitted as long as there is measurable disease outside of the radiotherapy port

⁃ Participants will be excluded from the study if any of the following criteria apply. The participant has/is:

• History of prior systemic treatment of SCLC

• Prior diagnosis of non-small cell lung cancer is excluded if the cancer is diagnosed \< 3 years prior to study entry. Additionally, the participant must be off all therapy for the NSCLC at the time of study entry.

• Mixed small cell and non-small cell histology

• Suspected metastatic cancer from other sites (i.e., those without a known or suspected lung primary diagnosis)

• Not willing to have additional blood samples collected