Notes: This article is about the decade that began in the year 40. Sometimes the '40s is used as shorthand for the 1940s, the 1840s, or other such decades in various centuries – see List of decades. In American vernacular, '40s' may also refer to forty ounce bottles of malt liquor.

Ribosomal particles are denoted according to their sedimentation coefficients in Svedberg units. The 40S subunit is the small subunit of eukaryotic 80S ribosomes. It is structurally and functionally related to the 30S subunit of 70S prokaryotic ribosomes. However, the 40S subunit is much larger than the prokaryotic 30S subunit and contains many additional protein segments, as well as rRNA expansion segments.