Background：

The protein encoded by this gene is a membrane glycoprotein that mediates cell-cell signaling and plays a critical role in the growth and development of multiple organ systems. An extraordinary variety of different isoforms are produced from this gene through alternative promoter usage and splicing. These isoforms are expressed in a tissue-specific manner and differ significantly in their structure， and are classified as types I， II， III， IV， V and VI. Dysregulation of this gene has been linked to diseases such as cancer， schizophrenia， and bipolar disorder (BPD).