Sequence alignment of human FL and KL proteins. The figure illustrates that both colony-stimulating factors are type I transmembrane proteins with short cytoplasmic domains; both are likely to be four helix bundle proteins (based on x-ray crystallography data in the case of M-CSF⁸²). The approximate positions of the four helices are shown. The vertical red lines show the locations of introns (to the nearest amino acid) within the genes33,93,95,104 and illustrate their common genomic structure and ancestral origin. Conserved cysteine residues are shaded in color to reflect the formation of proposed intramolecular disulfide bonds (3 in the case of FL and 2 in the case of KL). Possible sites for N-linked glycosylation are boxed. The alignment is based on the one originally proposed by Bazan78 for KL and M-CSF.