Fli-1, Friend Leukaemia Virus Integration 1 gene

Fli-1 protein is a member of the ETS family of DNA-binding transcription factors. Approximately 90% of Ewing's sarcoma/PNETs have a specific translocation, t(11;22)(q24;q12). This results in the fusion of EWS to Fli-1 and production of a fusion protein which is a more potent transcription factor than normal Fli-1. The EWS/Fli-1 fusion transcript is not specific to Ewing's sarcoma/PNET, having also been described in childhood soft tissue sarcomas with neural and myogenic differentiation3,4 and classic neuroblastoma5.

Both polyclonal (Fli-1p) and monoclonal (Fli-1m) antibodies are available.

Immunohistochemical expression

Immunoreactivity is nuclear.

Among small round cell tumours, it is specific to Ewing sarcoma/PNET and lymphoblastic lymphoma:

The sensitivity and specificity of polyclonal (Fli-1p) and monoclonal (Fli-1m) anti-Fli-1, CD99 and FISH with the EWSR1 (22q12) dual-colour break-apart probe have been compared6. In normal tissues, Fli-1p expression was restricted to haemopoietic and endothelial cells. However, Fli-1m was also expressed by the prostate, breast , colon and squamous epithelium.

In conclusion, Fli-1m is sensitive but lacks specificity for EWS/PNET. The combination of CD99 and Fli-1p is optimal.

• endothelial cells and vascular tumours2: