PCDH18 (protocadherin 18) is a non-clustered protocadherin that functions as a calcium-dependent cell adhesion molecule with critical roles in brain development and potential tumor suppression. As a member of the δ2 subgroup of non-clustered protocadherins, PCDH18 is predominantly expressed in the nervous system and exhibits region-specific expression patterns in cortical areas during early postnatal development and in mature brain structures including caudate putamen and hippocampal formation 1. The protein demonstrates homophilic/heterophilic cell-cell adhesion properties and interacts with mDab1 for intracellular signaling 1. PCDH18 plays important roles in neuronal circuit formation and maintenance, with haploinsufficiency potentially contributing to intellectual disability, as demonstrated in a patient with severe developmental delay, seizures, and brain malformations carrying a deletion encompassing only PCDH18 2. The gene is upregulated during early brain development (10-12 weeks gestation) alongside genes involved in sensory organ development 3. Beyond its neurological functions, PCDH18 acts as a tumor suppressor in colorectal cancer, where it is frequently silenced by promoter hypermethylation 4. Overexpression of PCDH18 suppresses cancer cell viability, colony formation, and migration, while its depletion promotes oncogenic behaviors through β-catenin pathway activation 4. PCDH18 deletions are also observed in aggressive pituitary adenomas and metastatic colorectal cancers 56.