Xenograft tumor progression is marked by tumor growth and KIT immunostaining of a spindle cell neoplasm. A. At 21.1 weeks, this P0 mouse was sacrificed for tumor passage. The primary tumor in the liver measured 8.5 × 7 × 6.5 mm on gross examination. B. Blinded review was performed by a pathologist. H&E staining confirmed the presence of a spindle cell neoplasm consistent with a GIST. This tumor appears to invade into the liver parenchyma. C. Positive KIT immunostaining was present in the xenografted tumor but not in the adjacent non-neoplastic liver.