Heavy metals are here treated as light metals and, in general, are not further considered. The heaviness of naturally occurring metals such as gold, copper, and iron may have been noticed in prehistory and, in light of their malleability, led to the first attempts to craft metal ornaments, tools, and weapons. All metals discovered from then until 1809 had relatively high densities; their heaviness was regarded as a singularly distinguishing criterion. From 1809 onwards, light metals such as sodium, potassium, and strontium were isolated. Their low densities challenged conventional wisdom and it was proposed to refer to them as "metalloids" (meaning "resembling
Poxvirus AX element late mRNA cis-regulatory element Poxvirus AX element late mRNA cis-regulatory element The Poxvirus AX element late mRNA family represents a "cis"-regulatory element present at the 3' end of poxvirus late ATI mRNA and is known as the AX element. The AX element is involved in directing the efficient production and orientation-dependent formation of late RNAs. It is likely that this element directs the endonucleolytic cleavage of the transcript. It has been shown that the F17R late mRNA transcript which is also cleaved is also likely to share a common factor in their mechanism despite a lack of any obvious similarity in its cis-regulatory RNA