Each of the subunits consists of two transmembrane helices and an extracellular loop. Calcium channels consist of six transmembrane helices. There are two transmembrane domains, each with six spans of alpha helices. Four alpha helices connect successive beta strands. Each domain contains 6 membrane spanning alpha helices. Ras contains six beta strands and five alpha helices. The reaction initiates the exchange of strands between two recombining DNA double helices. The fold may contain additional strands joined by short helices or coils. These macromolecules also coil into helices (of a different type) and crystallize. Molecules that tend to self-assemble into regular helices are often unwilling to assemble into crystals. The most common secondary structures are alpha helices and beta sheets. IL-12 is composed of a bundle of four alpha helices. An average armadillo repeat is composed of three alpha helices. The helices and their struts were installed. Multihelical versions with additional helices also occur.