CD99 and squamous cell carcinoma: CD99 has been reported to be a useful marker of AFX, but the positivity rate for CD99 has been reported to vary between 35% (Mirza B and Weedon D, 2005) and 73% (Monteagudo C et al. 2002) of AFX cases, although none of the squamous cell carcinoma cases have been reported to be positive for CD99 (Monteagudo C et al. 2002).