A systems-biology approach to identify Toxoplasma effectors that co-opt host cell transcription and their function in immune system suppression.

A single-celled parasite called *Toxoplasma gondii* has evolved a molecular toolkit of roughly 200 secreted proteins that it injects into host cells to hijack their genetic machinery, but scientists do not know which of these tools actually do the job. This matters because *Toxoplasma* infects about 30% of the global human population and any nucleated cell of any warm-blooded animal. Some strains cause severe disease in humans but not in other species, and vice versa. The difference likely lies in which effector proteins the parasite uses to suppress the host's immune response, but the specific proteins responsible for these transcriptional changes remain unknown. The researchers have developed a new technique called EffectorSEQ that can test many parasite mutants at once and correlate each with the host cell's resulting gene activity. They will use it to identify which of the ~200 effector proteins alter transcription in different cell types and species, then focus on those that promote anti-inflammatory immune environments favourable to parasite growth. This is fundamental science. Understanding how *Toxoplasma* co-opts host cell transcription across such a broad host range could eventually reveal new targets for drugs or vaccines, but the immediate goal is to map the parasite's molecular strategy for immune suppression.