The TxpA/RatA toxin-antitoxin system was first identified in Bacillus subtilis. It consists of a non-coding 222nt sRNA called RatA (RNA anti-toxin A) and a protein toxin named TxpA (Toxic protein A). RatA was discovered in intergenic regions of the B. subtilis genome, in a 728-nucleotide region between genes yqdB (later renamed TxpA) and yqbM. Initially, Affymetrix microarrays were used to detect transcription in this region, Northern blot experiments and mutation analysis were then employed to characterise the RNA transcript.