Like anatoxins, the saxitoxins (STX) are neurotoxic alkaloids which are also known as PSP's (paralytic shelfish poisons) due to their occurance and association with seafood. They block sodium channels in nerve cells, thus casuing their neurotoxic effects. There are a number of STX variants generally divided into groups based on their structure or organism of origin. The single sulphated STX's are known as gonyautoxins (GTX) and the doubly sulphated STX's are known as C-toxins. There are also decarbamyl STX's (dcSTX) and a group of STX variants, so far found only in Lyngbya wollei, known as Lyngbya-wollei-toxins (LWTX). STX's are highly toxic with LD50's as low as 10 µg kg-1 (i.p.) in mice. The STX family has the following general structure.