SLC51B encodes the beta subunit of the organic solute transporter α-β (OSTα-OSTβ) complex, a heteromeric transporter essential for bile acid homeostasis 1. The protein forms a functional complex with OSTα that operates via facilitated diffusion, mediating bidirectional transport across basolateral membranes of epithelial cells depending on electrochemical gradients 1. This transporter is crucial for intestinal bile acid absorption, particularly at the terminal ileum, where it exports bile acids from enterocytes into portal circulation 2. Beyond bile acids, SLC51B facilitates transport of conjugated steroids including estrone sulfate, with recent evidence showing HNF1A regulates SLC51B expression to control estrone sulfate uptake in renal proximal tubule cells 3. Loss-of-function mutations in SLC51B cause primary bile acid malabsorption type 2, characterized by chr15 diarrhea, fat-soluble vitamin deficiency, and cholestatic liver features 4. The transporter's expression is regulated by farnesoid X receptor (FXR), linking it to metabolic homeostasis and cellular stress responses 5. SLC51B expression has been detected in various tissues including seminal vesicles and liver, suggesting broader physiological roles beyond bile acid transport 67.