Soluble and transmembrane forms of human SCF. The primary proteolytic cleavage site of SCF²⁴⁸ in exon 6 is indicated by the arrow. Cleavage at this site generates the soluble form of SCF. The transmembrane form of SCF, SCF²²⁰, lacks the primary proteolytic cleavage site in exon 6. The 25 amino acid signal sequence (dotted lines) and the hydrophobic transmembrane domain (dark box) are also shown. As described in the text, murine SCF²⁴⁸ and murine SCF²²⁰ can also be cleaved at an alternative site in exon 7.