Other conformations are possible; A-DNA, B-DNA, C-DNA, E-DNA,[25] L-DNA (the enantiomeric form of D-DNA),[26] P-DNA,[27] S-DNA, Z-DNA, etc. have been described so far.[28] In fact, only the letters F, Q, U, V, and Y are now[update] available to describe any new DNA structure that may appear in the future.[29][30] However, most of these forms have been created synthetically and have not been observed in naturally occurring biological systems.[citation needed] There are also triple-stranded DNA forms and quadruplex forms such as the G-quadruplex and the i-motif.