Clear cell sarcoma of kidney (CCSK), light micrograph. CCSK is a rare malignant mesenchymal neoplasm that is seen predominantly in pediatric patients. Although rare, it is the second most common pediatric renal tumour after Wilms tumour. It is characterized by aggressive behaviour, late recurrences, and a tendency to metastasize to bones (hence the previous name - bone metastasizing renal tumour of childhood). It shows two specific, mutually exclusive genetic alterations. About 85% of cases show internal tandem duplications in BCL-6 coreceptor (BCOR). Another 10% of cases have a translocation t(10;17) creating the fusion gene YWHAE-NUTM2B/E. The remaining 5% carry neither genetic alteration. This image shows anaplastic variant of CCSK. It consists of bizarre hyperchromatic nuclei and atypical mitoses. This finding, seen in about 3% of cases of CCSK, may be focal or diffuse. The anaplastic foci are strongly P53+. Note that many nuclei are optically clear in this image.