Summary Androgen deprivation therapy (ADT) is effective in treating metastatic prostate adenocarcinoma (PADC), but all patients inevitably relapse with castrate resistant prostate cancer (CRPC). Most CRPCs remain dependent on androgen receptor (AR) signaling, but a significant fraction lack AR expression, become AR signaling independent, and aberrantly express neuroendocrine lineage markers (NEPC). The incidence of these NEPC variants has increased as more patients benefit from improved ADTs like enzalutamide and abiraterone acetate. This suggests increasingly stringent AR signaling blockade is driving development of NEPC. NEPC is aggressive and lethal, thus reflecting a growing clinical problem. Development of effective therapies is hampered by limited understanding of relevant molecular mechanisms. NEPC clearly arises from ARpos CRPC as they share clonal origin in patients that harbor both. We have determined that genetic inactivation of the RB1/TRP53 tumor suppressor genes cooperate to facilitate transformation of ARpos PADC to NEPC through derepression of epigenetic reprogramming factors. Inhibiting these reprogramming factors reverses NEPC transformation and restores ADT sensitivity, demonstrating that epigenetic changes are involved. We hypothesize Notch signaling within the tumor microenvironment suppresses the epigenetic reprogramming underlying NEPC transdifferentiation. This hypothesis has clinical ramifications as the pathway could conceivably be manipulated therapeutically to delay or reverse NEPC transformation, extending the duration of beneficial ADT clinical responses in some patients. This administrative diversity supplement application is proposed to support Mauricio Flores, a predoctoral student who will be involved in the parent R01 grant supported research program characterizing the role of NOTCH signaling in NEPC transdifferentiation. The research, mentoring, and career development plan proposes three research aims: 1) Identify organoid culture conditions sufficient to support NEPC transdifferentiation in vitro; 2) Test how Notch signaling affects NEPC transdifferentiation in vitro; 3) Identify stromal-cancer cell interactions that influence NEPC transdifferentiation in vitro. The long-term goal of this diversity supplement is to facilitate Mr. Flores' cancer research career development and to improve prostate cancer therapy by advancing mechanistic understanding of lineage plasticity as a mechanism of acquired therapeutic resistance.