Dysfunctional proteins drive the onset and progression of cancer. The protein CDK9 is a major player in cell cycle regulation. It modulates and alters the half-life of various downstream proteins such as the Androgen Receptor (AR) which regulates gene expression. A truncated and therefore dysfunctional version of AR represents the prime molecular target that drives castration-resistant prostate cancer (CRPC). This truncated version is considered “undruggable”. This image provides a first insight into the molecular details of a drug-like small molecule selectively binding to CDK9 and thereby provides the foundation for further drug development approaches.