RTEL1 and famililal cerebral cavernous malformations: In addition, the harmonin homology domain (HHD) (or harmonin-N-terminal domain) from whirlin (28), harmonin (29), cerebral cavernous malformation 2 (CCM2) (30), and regulator of telomere elongation helicase 1 (RTEL1) (31) was assigned to the αα-hub domain group (24) (Figs. 1E and 2E).