The standard perception of a restriction enzyme is of an endonuclease that recognizes a short palindrome of DNA and which cleaves both strands of the DNA at fixed locations in that sequence, in 'To whom correspondence should be addressed. a reaction that requires only Mg2+ ions as a cofactor [l]. The perception thus refers to the type I1 restriction enzymes, as opposed to the type I and the type 111 systems [2]. Many of the type I1 restriction enzymes conform to this perception [3]. Their recognition sites are often symmetrical 0 1999 Biochemical Society

The authors examined the occurrence of theoretically derived patterns of thinking in 74 agoraphobic participants as they drove alone or tolerated an enclosed place. During the increasingly scary tasks in a behavioral test hierarchy, participants responded to a periodic beep by stating aloud what they were thinking at that moment, yielding more than 1,800 tape-recorded statements. Content analyses revealed that participants were mainly preoccupied with their current anxiety (expressed in 29% of the statements) and with their self-efficacy (15%). Despite participants' mounting feelings of anxiety, fewer than 1% of their statements expressed a thought of danger or an anticipation of future anxiety or panic. The rarity of danger thoughts poses an explanatory challenge for all cognitive theories of phobia and especially for the perceived danger theory of A. T. Beck (1976) and A. T. Beck, G. Emery, and R. L. Greenberg (1985).

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