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 [https://en.wikipedia.org/wiki/Selenomethionine 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 [https://en.wikipedia.org/wiki/Multi-wavelength_anomalous_dispersion MAD in Wikipedia).