Ligand for members of the frizzled family of seven transmembrane receptors. Acts in the canonical Wnt signaling pathway by promoting beta-catenin-dependent transcriptional activation (By similarity). In some developmental processes, is also a ligand for the coreceptor RYK, thus triggering Wnt signaling (PubMed:15454084, PubMed:16116452). Plays an essential role in the development of the embryonic brain and central nervous system (CNS) (PubMed:2202907, PubMed:16116452). Has a role in osteoblast function, bone development and bone homeostasis (By similarity). Forms a soluble 1:1 complex with AFM; this prevents oligomerization and is required for prolonged biological activity. The complex with AFM may represent the physiological form in body fluids (By similarity). Interacts with PORCN (PubMed:10866835). Interacts with RSPO1, RSPO2 and RSPO3 (PubMed:16543246). Interacts with WLS (PubMed:19841259). Testis and mid-gestational embryos. In the testis, detected only in postmeiotic germ cells undergoing differentiation from round spermatids into mature spermatozoa. In the embryos, expression is restricted to the developing CNS in regions of the neural tube other than the telencephalon. Expressed in osteoblast; expression levels increase with advancing osteoblast differentiation. Expressed in the brain, femur, spleen, and hematopoietic bone marrow. Accumulates throughout the neural plate at the anterior head folds of the 9 day embryo but only at its lateral tips in more posterior regions. Following neural tube closure, expression is restricted to specific regions of the dorsal wall of the brain ventricles and spinal cord, the ventral wall of the midbrain and the diencephalon, and the lateral walls of the neuroepithelium at the midbrain-hindbrain junction. Palmitoleoylation is required for efficient binding to frizzled receptors. Palmitoleoylation is necessary for proper trafficking to cell surface (By similarity). Depalmitoleoylated by NOTUM, leading to inhibit Wnt signaling pathway (By similarity). Important perinatal lethality, due to defects in brain development. After 14.5 dpc, embryos display dramatic malformations of the mesencephalon and metencephalon, and especially the cerebellum. They show mild midbrain hydrocephaly by 17.5 dpc. Only one out of ten live-born pups survives more than 30 days; these mice respond normally to light, sound, smell and touch, but display severe ataxia. Many mouse mammary tumors induced by mouse mammary tumor virus (MMTV) contain a provirus integrated into a host cell region which has been named INT-1 (now Wnt1). Belongs to the Wnt family. A palmitoylation site was proposed at Cys-93, but it was later shown that this cysteine is engaged in a disulfide bond.