- IFAP syndrome is caused by deficiency in MBTPS2, an intramembrane zinc metalloprotease essential for cholesterol homeostasis and ER stress response. Oeffner, F., Fischer, G., Happle, R., König, A., Betz, R.C., Bornholdt, D., Neidel, U., Boente, M.C., Redler, S., Romero-Gomez, J., Salhi, A., Vera-Casaño, A., Weirich, C., Grzeschik, K.H. Am. J. Hum. Genet. (2009)