Lineage plasticity (LP)—most commonly exemplified by loss of androgen receptor (AR) signaling and switch from a luminal to an alternate differentiation program—is now recognized as a critical determinant of lethality. Most efforts in the field are focused on factors that promote terminal differentiation of specific LP subtypes such as neuroendocrine prostate cancer (NEPC). However, LP is a continuum, ranging from AR activity-low tumors with persistent AR expression but low AR signaling to those with alternate differentiation programs. Factors involved in the initiation and progression of LP have not been identified. The clarification of such factors is essential because there are no effective therapies for prostate tumors that have undergone LP. In this proposal we seek to target key factors involved in early LP events to prevent LP from occurring. Using an integrative genomic analysis of metastatic castration-resistant prostate cancer patient biopsies, we determined that key transcriptional regulators are upregulated early on in the LP continuum in AR activity- low tumors and that these factors progressively increase in terminally differentiated LP subtypes. Importantly, gain of function studies of these transcriptional regulators in AR-driven tumors and loss of function studies in tumors representing the continuum of LP demonstrate that these regulators block AR signaling and cooperate with other factors to promote specific alternate differentiation programs. Targeting these transcriptional regulators or their key epigenetic cofactors reduces survival of LP tumor cells in vitro and in vivo with good animal tolerability, strongly suggesting we have a promising approach to block LP. We hypothesize that key transcriptional regulators we have identified are early drivers of LP that control prostate cancer cell fate decisions by: 1) repressing AR signaling and luminal differentiation and 2) activating alternate differentiation programs in concert with key cofactors.