Due to the relatively short half-life of its starting isotope neptunium-237 (2.14 million years), the fourth chain, the neptunium series with "A" = 4"n" + 1, is already extinct in nature, except for the final rate-limiting step, decay of bismuth-209. Traces of 237Np and its decay products still do occur in nature, however, as a result of neutron capture in uranium ore. The ending isotope of this chain is now known to be thallium-205. Some older sources give the final isotope as bismuth-209, but it was recently discovered that it is very slightly radioactive, with a half-life of .