Craig Taylor and colleagues (Dec 10, p 2019) 1 estimate that about 150 human embryonic stem-cell (hESC) lines would be needed for most of the UK population, and that as few as ten might be sufficient if one were to prospectively identify cell lines that could serve a larger number of patients, such as lines homozygous for common HLA types. This important study provides a basis for realising the concept of stem-cell banking and the criteria according to which cell lines should be deposited in such a bank. We would like to add the following points.

We have determined the HLA type for over half the hESC lines eligible for US federal support (unpublished data). No line homozygous for all HLA loci has been derived, but lines homozygous at some loci are available. Further examination of the limited data available does not support the premise (as expected) that the allelic frequency matches that predicted from the UK donor data set. 1 This finding suggests that the lower estimates of the number of cell lines required is not supported by the current lines available. A substantial challenge in the identification of matched donors is the diversity of the HLA system. Almost 1500 alleles have been identified at 12 HLA loci and additional alleles are still being discovered. 2 The hESC lines currently available in the USA were derived at only a few institutions and are unlikely to reflect the ethnic diversity of the US population. For example, it is unclear (given privacy restrictions) whether any lines were derived from Hispanic, African American, or Asian donors. These ethnic groups constitute significant minorities in the USA. Problems finding suitable HLA-matched donor organs, cord blood, and bone marrow