Table 1.

Categorizing the mode of action and the location of regulatory elements with examples from the E. coli system.

	cis	trans
local	operators, promoters	neighbouring transcription factors
	lac operator	lacI
distant	enhancers	distant transcription factors
	Shh ZPA enhancera	Crp

^aIn the case of distant/cis regulators, there are no good examples in the E. coli system, so we report one example from mammals in the sonic hedgehog zone polarizing activity enhancer (Shh ZPA enhancer) which is a cis factor that regulates sonic hedgehog from up to 1 Mb away (Lettice et al. 2002; Jeong et al. 2006).