The NHL repeat, named after ncl-1, HT2A and lin-41, is an amino acid sequence found largely in a large number of eukaryotic and prokaryotic proteins. For example, the repeat is found in a variety of enzymes of the copper type II, ascorbate-dependent monooxygenase family which catalyse the C-terminus alpha-amidation of biological peptides.^[2] In many it occurs in tandem arrays, for example in the RING finger beta-box, coiled-coil (RBCC) eukaryotic growth regulators.^[3] The arthropod 'Brain Tumor' protein (Brat; Q8MQJ9) is one such growth regulator that contains a 6-bladed NHL-repeat beta-propeller.^[1][4]

The NHL repeats are also found in serine/threonine protein kinase (STPK) in diverse range of pathogenic bacteria. These STPK are transmembrane receptors with an intracellular N-terminal kinase domain and extracellular C-terminal sensor domain. In the STPK, PknD, from Mycobacterium tuberculosis, the sensor domain forms a rigid, six-bladed b-propeller composed of NHL repeats with a flexible tether to the transmembrane domain.

The NHL repeat has also been used to design a family of fully symmetrical 6-blade beta-propeller proteins called "Pizza".^[5] These proteins can also be engineered to bind mineral nanocrystals.^[6]