This family consists of several photosystem II reaction centre X protein (PsbX) sequences from both prokaryotes and eukaryotes.

1: Proteomic analysis of a highly active photosystem II preparation from the cyanobacterium Synechocystis sp. PCC 6803 reveals the presence of novel polypeptides. Kashino Y, Lauber WM, Carroll JA, Wang Q, Whitmarsh J, Satoh K, Pakrasi HB; Biochemistry 2002;41:8004-8012. PMID:12069591