Selenomethionine (designated MSE in the Protein Data Bank) is methionine in which the sulfur atom is replaced by selenium. Methionine is one of the 20 standard amino acids (see Methionine in Wikipedia). Selenomethionine occurs naturally (see Selenomethionine in Wikipedia) but is also introduced artificially into proteins prior to crystallization in order to solve phases by multi-wavelength anomalous dispersion (MAD; see MAD in Wikipedia).
4hik is an example of a crystallographic structure solved by MAD. The crystallized protein contained 143 amino acids taken from the middle of the 555 full-length protein. The wild-type sequence of the crystallized fragment contained three methionines, all of which were replaced by selenomethionine.