The trial court erred in excluding plaintiff’s expert witness from testifying about the causal connection between the mold in her apartment and her physical ailments.  The trial court exercises an important gatekeeping role in ensuring that an expert’s opinion is based on reliable evidence and sound reasoning.  However,  the factual basis of an expert opinion goes to the credibility of the testimony, not the admissibility. An inspection service had found large amounts of toxic mold in plaintiff’s apartment.  To prove causation, an expert could use epidemiological studies to show a statistical link between the mold and illness or, alternatively, use differential diagnosis.  Here, plaintiff’s expert, a medical doctor, used both approaches.  As plaintiff’s treating physician, the expert was in the best position to determine the cause of her illness, and his causation analysis was supported by recent scientific studies linking toxic mold to upper respiratory illnesses.  And courts have generally agreed with those studies.