The mammalian small ribosomal subunit (40S) is the smaller subunit of the mammalian 80S ribosomes, with the other major component being the large ribosomal subunit (60S). The 40S subunit consists of a 18S ribosomal RNA and ~32 ribosomal proteins. The 40S subunit contains the decoding center which monitors the complementarity of tRNA and mRNA in protein translation.