An observational study by Ried et al. [46] involving 600 CTCs tests on 542 patients showed that these cells were detected in 100% of cancer patients (n = 277) and 50% of asymptomatic patients with risk factors (132 out of 265), with follow-up scans identifying early cancerous lesions in 20% of screened patients and early prostate cancer in 50% of males with normal prostate specific antigen (PSA) levels but positive CTCs results, highlighting the value of CTCs analysis for early detection and monitoring. This evidence concerns the gene KLK3 and prostate cancer.