Axin-1 is constitutively expressed and acts as a negative regulator of Wnt expression. It has been demonstrated that mouse Wnt, Wnt3a, is secreted by multiple cell lines and can act and as a growth factor for stem cells. Secreted Wnt3a is highly hydrophobic because it is palmitoylated on an essential cysteine C77; the palmitoylated state of Wnt3a is essential for stem cell signaling. Incubation of stem cells with Wnt3a in tissue culture medium resulted in the increased number of mouse hematopoietic stem cells expressing "primitive" phenotype.