In 1986, convincing evidence was provided that selenocysteine (Sec) was incorporated co-translationally. Moreover, the codon partially directing its incorporation in the polypeptide chain was identified as UGA also known as the opal termination codon. Different mechanisms for overriding the termination function of this codon have been identified in prokaryotes and in eukaryotes. A particular difference between these kingdoms is that cis elements seem restricted to the neighborhood of the UAG codon in prokaryotes while in eukaryotes this restriction is not present. Instead such locations seem disfavored albeit not prohibited. 
In 2003, a landmark paper described the identification of all known selenoproteins in humans: 25 in total. Similar analyses have been run for other organisms.
The UAG codon can translate into pyrrolysine (Pyl) in a similar manner.