Illustration of messenger RNA (mRNA, bottom left corner) binding to transfer RNA (tRNA, top left corner). A strand of mRNA is shown leaving the cell nucleus (upper right) and binding to sequences of three nucleotides bases (codons) on a strand of tRNA. Binding between the codons of mRNA and tRNA is normally constrained because the bases adenine (A), cytosine (C), guanine (G) and uracil (U) (shown as rods on the RNA strands) can only combine in certain combinations; A to U and C to G. Abnormalities in this sequencing can result in mistakes in the transcription of the genetic code into proteins.