PCP4 (Purkinje cell protein 4) is a 7.6 kDa calcium-binding regulatory protein that functions as a modulator of calmodulin activity 1. Originally identified in cerebellar Purkinje cells, PCP4 exhibits tissue-specific expression patterns and plays diverse roles in cellular processes 2. In cancer contexts, PCP4 demonstrates complex functions: it promotes migration, invasion, and adhesion in breast cancer cells by reducing E-cadherin expression and enhancing filopodia formation 3. PCP4 also upregulates aromatase gene expression through CYP19A1 promoter I.1 in certain breast cancer cells, potentially supplying estrogens in the tumor microenvironment 4. Conversely, PCP4 acts as a tumor suppressor in prostate cancer by stabilizing CAMKK2 protein and inhibiting progression through the Ca2+/CAMKK2/AMPK/AR signaling axis 5. In adrenal tissues, PCP4 upregulation in aldosterone-producing adenomas fosters cell growth via AKT and AMPK pathways 6. Neurologically, PCP4 suppresses neuronal differentiation by downregulating neurite outgrowth through transcriptional regulation of Ascl1 and NeuroD1 1, and genetic variants affecting PCP4 splicing have been linked to amyotrophic lateral sclerosis, with PCP4 being crucial for maintaining dendritic spine density 7.