The 40S subunit is a component of the ribosome, which is essential for protein synthesis in cells. It is the smaller of the two ribosomal subunits in eukaryotes, the other being the 60S subunit. The 40S subunit is responsible for decoding messenger RNA (mRNA) and ensuring that the correct amino acids are added to the growing protein chain. During the process of translation, the 40S subunit binds to the mRNA and scans for the start codon, where protein synthesis begins. This subunit also interacts with transfer RNA (tRNA) molecules, which bring the appropriate amino acids to the ribosome, facilitating the assembly of proteins.