This paper studies the construction lo que es as a cleft sentence in 108 sociolinguistic interviews from the «Corpus sociolingüístico de la ciudad de México» (CSCM), in the context of all lo que found in the corpus. Syntactic and pragmatic properties are analysed and its social distribution is established. The removability of cleft construction and the character of ser as a weak verb are especially relevant. We argue that lo que es is a marker of informative prominence.