A
molecular assembler, as defined by
K. Eric Drexler, is a "proposed device able to guide chemical reactions by positioning reactive molecules with atomic precision". A molecular assembler is a kind of
molecular machine. Some biological molecules such as
ribosomes fit this definition. This is because they receive instructions from
messenger RNA and then assemble specific sequences of
amino acids to construct
protein molecules. However, the term "molecular assembler" usually refers to theoretical human-made devices.