This is why, only little information about the cyst fluid is available and the hypotheses for cyst formation include necrobiotic degradation of tumor tissue, active secretion of proliferative factors by the tumor cells, or mere trapping of cerebrospinal fluid (CSF).5–7 Depending on the way of cyst formation, the content of the cyst fluid is supposed to include apoptotic pathway proteins, e.g. TNF, Fas, or caspases, high concentrations of specific secreted proteins such as VEGF or TGF-beta, or a protein content very similar to that of CSF. Here, FAS is linked to cyst.