Words in a field are assumed to be connected by a Boolean OR statement unless otherwise specified. One way to connect two words is by enclosing them in quotation marks. For example, the search
signal transduction
will return articles which include either the term signal or the term transduction (or both). A phrase search enclosed in quotation marks:
"signal transduction"
will only return articles where the term "transduction" immediately follows the term "signal"; articles containing only signal, only "transduction", or even "transduction signal" are not returned.