Alternative splice forms of the Csf1 receptor (c-fms). (a) Genomic alignment (mm5; chr18:61616977..61647364) of full-length and variant receptors displaying exon structure and peptide features. Also shown are subcellular localizations of variant receptors transiently expressed in HeLa cells: (b) full-length Csf1r (GenBank:AK076215); (c) Tethered1 (GenBank:AK155565); (d) Tethered3 (GenBank:AK171543); and (e) Secreted (GenBank:AK171241). Tethered forms are produced by exon skipping (Tethered1; c), termination within an intron (Tethered2), and a mutually exclusive alternative 3' exon (Tethered3; d). Tethered forms 1 and 3 exhibit similar localizations to that of the full-length receptor (panel b; cell surface and perinuclear puncta). The form lacking the transmembrane (TM) domain is absent from the cell surface and displays a secretory pathway-like localization.