In my experience with serum-based media in expanding MSC, I’ve found that reducing serum does seem to improve variability issues. However, improvement would depend on the extent of serum reduction as well as the composition of the substituted defined formulation. With this said, serum reduction does have some issues. Generally speaking, the problem with serum reduction in MSC is that a defined formulation that is optimized for one lot of serum may have inadequate levels of components when paired with a different lot of serum. Therefore, if MSC performance variability is an issue, I would recommend eliminating serum all together.