Protein layers (1) repeating regularly in two dimensions comprise protein protomers (2) which each comprise at least two monomers (5), (6) genetically fused together. The monomers (5), (6) are monomers of respective oligomer assemblies (3), (4) into which the monomers are assembled to assembly of the protein layer. The first oligomer assembly (3) belongs to a dihedral point group of order O, where O equals (3), (4) or (6) and has a set of O rotational symmetry axes of order (2). The second oligomer assembly (4) has a rotational symmetry axis of order (2). Due to the symmetry of the oligomer assemblies (3), (4), the rotational symmetry axes of each second oligomer assembly (4) is aligned with one of said set of O rotational symmetry axes of a first oligomer assembly (3) with (2) protomers being arranged symmetrically therearound. Thus, an 2-fold fusion between the oligomer assemblies (3), (4) is produced and the arrangements of the rotational symmetry axes of the oligomer assemblies (3), (4) cause the protein layer to repeat regularly. The protein layer has many uses, for example to support molecular entities for biosensing, x-ray crystallography or electron microscopy.