In addition to heritability, another desired feature of transgenic experiments is often the ability to control the cell- and tissue-specific expression of a gene of interest. To this end, significant progress has been made investigating

the key elements that drive this specificity in particular organisms. For example, in S. stercoralis, 5′ and 3′ regulatory sequences were shown to play important roles in driving tissue-appropriate transgene expression. Through the use of a specific promoter and 3′ UTR sequence from the S. stercoralis gene Ss era-1, GFP expression was limited to intestinal cells of developing F1 progeny (96). In a subsequent study by the same group, they further demonstrate that other promoter sequences derived from the parasite itself leads to tissue-specific expression that was dependent on the promoter sequence used (98). One EPZ 6438 outcome of these findings is the development of collections of modular vectors to enable regulated expression

of a gene of interest. One such collection is available to the research community for use in S. stercoralis (http://www.addgene.org). The ability to select for transgenic parasites will be extremely important for unambiguous interpretation of experimental GDC-973 results. Whilst there is much known about the sensitivity of protozoan parasites to a range of antibiotics and other drugs enabling the development of selectable marker genes that confer Phospholipase D1 drug resistance (99–107), significantly less is know about the sensitivity of parasitic helminths to similar compounds. For example, whilst

Strongyloides ransomi are apparently somewhat sensitive to hygromycin (108), none of the other commonly used antibiotics are known to be effective against Strongyloides sp. In lieu of the identification of suitable antibiotics, fluorescent markers of gene expression allow for the selection of transgenic parasites by standard methodologies such as flow cytometry (109). Nevertheless, the development of additional selectable markers will be extremely important for the future development of parasitic helminth transgenesis. In 2002, Hussein et al. (110) reported the establishment of an effective knock-down of acetylcholinesterase A, B and C in Nippostrongylus brasiliensis by soaking adult parasites in long dsRNA-containing medium. The gene knock-down was reflected by reduced transcript and protein levels as well as a decrease in enzyme activity in vitro. Following this first publication, RNA interference has only been applied to a limited number of clade III and V parasitic nematodes of animals, including Ascaris suum, B. malayi, L. sigmodontis, Onchocerca volvulus, Haemonchus contortus, N. brasiliensis, Ostertagia ostertagi, Trichostrongylus colubriformis and recently, Heligmosomoides polygyrus (see Table 2).