C1QL1 (complement C1q like 1) is a secreted protein with pleiotropic functions spanning synaptic regulation and cancer biology. In the central nervous system, C1QL1 regulates excitatory synapse formation in hippocampal neurons 1 and maintains synaptic connectivity through calcium-modulated assembly of C1ql1-BAI3 complexes in the synaptic cleft 2. In the cochlea, C1QL1 regulates auditory nerve fiber growth via the BAI3-ELMO1-DOCK180-RAC1 integrin pathway 3. Beyond neurobiology, C1QL1 exhibits tissue-specific functions: it promotes endothelial cell angiogenesis through ERK1/2 pathway activation 4. Regarding cancer, C1QL1 displays context-dependent roles. In breast cancer, C1QL1 acts as a tumor suppressor, with reduced expression due to promoter methylation; overexpression inhibits proliferation and metastasis while promoting apoptosis through the HSP90α/VCP-endoplasmic reticulum stress/unfolded protein response axis 5. Conversely, in glioblastoma, C1QL1 functions as a glioma-promoting factor during malignant transformation 6, and in renal clear cell carcinoma, elevated C1QL1 promotes tumor progression via the YAP-epithelial-mesenchymal transition pathway 7. In thyroid cancer, C1QL1 overexpression predicts extrathyroidal extension 8. These findings establish C1QL1 as a multifunctional protein with both protective roles in neural development and context-dependent oncogenic or tumor-suppressive properties.