Publisher Summary This chapter describes the molecule Stem Cell Factor (SCF), is involved in the development of haematopoietic, gonadal and pigment cell lineages. This is also known as “Mast Cell growth factor” (MGF), kit ligand, steel factor. The cross-reactivity indicates that there is 81% homology between human and mouse SCF. Human SCF has very little activity on mouse cells whereas rat SCF is active on human cells. The sources and bioassays of the molecule are discussed. The physiochemical properties of SCF are summarized in a table for human and mouse and the 3-D structure is available. The gene structure is schematically illustrated. The amino acid sequence for human SCF (Accession Code: SwissProt P21583) and mouse SCF (Accession code: SwissProt P20826) is provided. The structure of SCF receptor is illustrated schematically and the distribution is given. The physiochemical properties of the SCF receptor (c-kit) are given in a table. The signal transduction and the chromosomal location are provided along with the amino acid sequence for human SCF receptor (Accession code: SwissProt P10721) and mouse SCF receptor (Accession code: SwissProt P05532) are provided. A list of further resources concludes the chapter.