The presence of ≥4 CXCR4-positive CTCs was a poor prognostic factor for OS in SCLC patients with extensive disease (Kaplan Meier, Log Rank, p = 0.041, HR = 5.01; Figure 6a), compared to SCLC patients with ≤3 CXCR4-positive CTCs (4.5 months with range 1.6–7.4 vs. 11.4 months with range 8.5–14.4). Here, CXCR4 is linked to small cell lung carcinoma.