Transfer RNAs (tRNAs) are adaptor molecules that serve as the physical link between the nucleotide sequence of mRNA and the amino acid sequence of proteins. In 1973, structure of phenylalanine tRNA was the first large, polynucleic acid to be elucidated crystallographically first by Sung Hou Kim and his coworkers and subsequently by Aaron Klug and his coworkers. The two sculptural representations of phenylalanine tRNA were commissioned by Sung Hou Kim who participated in their construction. The larger of these is nickel-plated steels and stands 55 inches tall. The smaller tRNA is made of polished stainless steel and stands 44 inches tall.