<h4>Introduction</h4>Treatment-refractory metastatic castration-resistant prostate cancer is a heterogeneous disease classified into androgen receptor-high prostate cancer, androgen receptor-low prostate cancer, amphicrine prostate cancer co-expressing androgen receptor and neuroendocrine genes, double-negative prostate cancer lacking androgen receptor and neuroendocrine gene expression, and small cell or neuroendocrine prostate cancer without androgen receptor activity. The gene discussed is AR; the disease is prostate cancer.