Cancer stem cells (CSCs) are implicated in tumor initiation, development and recurrence after treatment, and have become the center of attention of many studies in the last decades. Therefore, it is important to develop methods to investigate the role of key genes involved in cancer cell stemness. Gastric cancer (GC) is one of the most common and mortal types of cancers. Gastric cancer stem cells (GCSCs) are thought to be the root of gastric cancer relapse, metastasis and drug resistance. Understanding GCSCs biology is needed to advance the development of targeted therapies and eventually to reduce mortality among patients. In this protocol, we present an experimental design using a conditional knockdown system and an adapted sphere formation assay to study the effect of clusterin on the stemness of patient-derived GCSCs. The protocol can be easily adapted to study both in vitro and in vivo function of stemness-associated genes in different types of CSCs.