Abstract: There is provide by the invention a method of diagnosing a human female suspected of being an symptomatic carrier of X-linked agammaglobulinemia (XLA). The method comprises sampling B cell lymphocytes of said female; and determining whether at least 95% of said B cell lymphocytes exhibit inactivation of the same chromosome; whereby said X chromosome inactivation is diagnostic of XLA. The invention further provides a method of screening a human female suspected of being an asymptomatic carrier of X-linked severe combined immunodeficiency (XSCID). The method comprises sampling the B or T cell lymphocytes of said female; determining whether at least 85% of said lymphocytes exhibit inactivation of the same chromosome; whereby said X chromosome inactivation is associated with XSCID.

Abstract: A process for determining the presence of a particular nucleic acid sequence in a test sample comprising(a) chemically modifying nucleic acids in the test sample either to introduce a label or a reactive site in a manner that supports their hybridizability,(b) contacting under hybridization conditions the chemically modified sample nucleic acids with a hybridizable nucleic acid probe which either, when the sample nucleic acids have been modified to introduce a label, carrys a reactive site or, when the sample nucleic acids have been modified to introduce a reactive site, is labeled,(c) contacting the solution resulting from step (b) with a immobilized form of a reactive partner to the reactive site to form a stable bond with the reactive site on the sample nucleic acids or the probe, respectively,(d) separating the resulting immobilized fraction from the remaining solution, and(e) determining the presence of the label in the separated immobilized fraction or a decrease in the label in the remaining solution.