Aim: We aimed to evaluate (immunohistochemically) the YAP expression in breast cancer patients undergoing neoadjuvant chemotherapy and to clarify the relationship between the molecular characteristics, treatment response and survival data and the YAP expression, and hence, to clarify the prognostic significance. Methods: One hundred and four patients who were admitted to Erciyes University Faculty of Medicine after the approval of the Ethics Committee and were diagnosed with Breast Cancer between 2015-2020 and underwent NeoAdjuvant Chemotherapy were included in the study. The diagnoses of these patients were determined by any of the methods of Breast USG, Mammography, Breast MRI and, if necessary, PET/CT, and among pathological samples, Estrogen Receptor (ER), Progesterone Receptor (PR), Human Epidermal Growth Receptor-2 (HER2) and Ki-67 Expression are routinely stained immunohistochemically. In this study, existing immunohistochemical markers were reviewed and also, the relationship of YAP with these biological markers was evaluated by using immunohistochemistry and its effect on prognosis has been investigated. Results: The average age of the patients was 52.37. While YAP was positive in 78 patients (75%), it was negative in 26 patients (25%). In the evaluation after neoadjuvant therapy, pathological complete response (Miller Payne Grade 5 response) in 28 patients (26.9%), relapse in 6 patients (5.8%), and exitus in 6 patients (5.8%) were detected. In the pathological evaluation, invasive Ductal Carcinoma was the most common one observed in 88 patients (84.6%). As a result of the statistical evaluation, no significant result was obtained between the parameters and YAP negative / positive. Conclusion: As a result of staining with additional YAP in patients who were diagnosed with breast cancer and routinely stained with ER, PR, Cerb B2 and Ki-67 in pathology samples, we could not reach a result that would contribute positively to survival. Longer studies to be conducted prospectively will be meaningful.