Passaging hPSCs as aggregates have been shown to allow the long-term expansion of many different cell lines while maintaining a normal karyotype. It is possible to passage human ES and iPS cells as single cells; however, it has been demonstrated that this practice can place unwanted selective pressure on cell populations and could lead to genetic aberrations in the culture.4,5 Therefore, if you perform single-cell passaging of human ES or iPS cells in any culture medium, check the karyotype frequently to ensure that the culture has retained a normal karyotype.