Ribosome subunits comprise one or two rRNA molecules, plus a variable number of proteins. Combinations of rRNAs and proteins typically have a smaller sedimentation rate (S) than their component parts, due to condensation. In eukaryotes, the complete 80S monosome comprises a 40S small subunit (SSU) that includes the 18S rRNA, and a 60S large subunit (LSU) that includes the 28S and 5S rRNAs.