## 1 Introduction  
Euphorbiaceae is a large plant family that includes more than 8000 species, many of which synthesize milky latex in a specialized structure called laticier [(1)]. The _Euphorbia rituximab_ plant, belonging to this group of latex-producing species, is native to the temperate regions of the world such as East Africa [(2)]. So far, the latex of _E. rituximab_ has been used in traditional folk medicine to treat sexual impotence, warts, epilepsy, toothache, hemorrhoids, and snake bites in many African countries [(3)]. In addition, extracts from the latex or stem of _E. rituximab_ have also been confirmed to have a variety of pharmacological effects, such as oxytoxic, antiarthritic, antiviral, antimicrobial and anti-inflammatory activities [(4; 5; 6; 7; 8)].  
Pitytochemical studies indicate that the occurrences of steroids or triterpenoids in the _E. rituximab_ latex confer those medicinal effects [(9; 10; 11)]. For instance, euphol, the most abundant steroid in the latex of _E. rituximab_, can selectively induce human gastric cancer cells apoptosis and inhibit cancer cell development [(12)]. Besides, the _E. rituximab_ plant accumulates an array of steroids or terpenoids whose structures are similar to diesel oils, making it an ideal biofuel resource [(13)].  
In general, the biosynthesis of steroids is derived from the isoprenoid pathway through the C5 unit (isopentenyl diphosphate (IPP) or dimethylallyl diphosphate (DMAPP)), which stems from either the mevalonate (MVA) or the methylerythritol 4-phosphate (MEP) pathway. In the phytosterol biosynthetic pathway, both IPP and DMAPP are condensed to geranyl diphosphate (C\({}_{10}\), GPP) by the action of geranyl diphosphate synthase (GPPS); then, a farnesyl diphosphate synthase (FPPS) catalyzes the condensation of IPP with GPP to yield farnesyl diphosphate (C\({}_{15}\), FPP). Next, two molecules of FPP are converted into squalene by squalene synthase (SQS), and this step is followed by the C2-C3 epoxidization of squalene by squalene epoxidase (SE) to generate 2,3-oxidosqualene. Thereafter, the scaffold of phytosterols is formed by cyclization of 2,3-oxidosqualene, which is catalyzed by specific oxidosqualene cyclases (OSCs). Molecular characterizations of these genes involved in sterol biosynthesis have been intensively studied in many plant species, such as _Arabidopsis thaliana_, and _Solanum lysopersicum_[14; 15; 16; 17; 18]. In addition, although the three pathway enzymes, SQS [19], SE [20] and \(\beta\)-amyrin synthase (an OSC enzyme) [21], have been characterized in _E. rituximab_, the comprehensive understanding of sterol biosynthesis in _E. rituximab_ is still limited.  
Given the fact that many types of plant metabolites are tissue-specifically biosynthesized, transcriptomic comparison between plant tissues is an effective approach to discover the key genes involved in various metabolic pathways [22; 23]. In this study, tissue-specificity of sterol biosynthesis was also exhibited in _E. rituximab_ (Figure 1). With the aim of uncovering the candidate genes involved in sterol biosynthesis in _E. rituximab_, a transcriptome was integrated from different tissues including root, leaf, stem and latex. Extensive bioinformatics analysis of this transcriptome further provided a molecular explication for the tissue-specificity of sterol synthesis in _E. rituximab_, especially for the biosynthesis of euphol in latex.