Identification of X linked mental retardation (XLMR) genes that can only be broadly localised by linkage analysis will ultimately depend on systematic screening of many probands for mutations in many candidate genes. This would be more efficiently performed by analysis of mRNA (or illegitimate transcripts) by reverse transcriptase-polymerase chain reaction (RT-PCR). A scheme is proposed that associates standardized reporting of XLMR families, including small families that would not by themselves yield statistically significant linkage information, and deposit of a lymphoblastoid cell line for one proband of each family to an accessible repository.