Selenomethionine
Selenomethionine 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).