In adult mouse, GNE mRNA expression is highest in the liver followed by the lung and kidney while Sia concentration measured in the spleen, brain and lung is considerably greater than in the liver or kidney. In adult rats, considerable GNE Neoandrographolide activity has been measured in the liver, salivary gland and small intestinal mucosa while in other tissues including spleen, brain and lung the GNE activity is very low. Interestingly, GNE activity in the liver of rats and guinea pigs has been reported to be low early in life during the post-natal suckling period when sialylation is considered essential for normal development and to rise significantly thereafter. The brain is the major site of Sia display and has been suggested to depend on an exogenous supply during growth and development. Muchmore also reported that in the colon and thymus and to a lesser extent in the small intestine of the neonatal rat there is a significant albeit transient increase in Sia display in the days immediately following birth. Thus as for the brain the early postnatal colon might need an exogenous supply of Sia to cope with its high needs. Such results have lead to speculation that during the neonatal suckling period de novo Sia production may not be sufficient to meet the needs of all tissues in the rapidly developing newborn and that Sia could serve as a conditionally essential nutrient for the suckling neonate. The uptake of Sia from an exogenous, say dietary, source is thought to initiate at the plasma membrane through endocytosis. Once intracellular the material within the endocytic vesicle is delivered to the lysosome where bound Sia is removed by a resident neuraminidase and then transported out into the cytosol by the Sia transporter sialin. Cytosolic Sia can then be activated through conjugation to CMP after which it is suitable to be used by Golgi-resident sialyltransferases to Tectorigenin decorate glycoproteins and glycolipids. Alternatively, Sia was also proposed to be catabolized to N-acetylmannosamine and pyruvate. Human and rodent milk is a rich source of Sia in the form of sialyloligosaccharides and thus it may be a valuable Sia supply for the newborn.