Abstract: Deamidation is usually viewed as a post-translational modification that sets an expiration date on proteins. Among apoptosis regulators of the Bcl-2 family, Bcl-xL shows a unique eligibility to be either singly or doubly deamidated. The inventors therefore analysed Bcl-xL deamidation state in platelets from mice models where platelets lifespan was manipulated. In parallel, the inventors compared human platelets obtained at steady state from healthy controls, to platelets newly synthesized after recovery from acute thrombocytopenia: they found that while expression levels of Asn52 monodeamidated Bcl-xL remains unchanged, Asn52Asn66 doubly-deamidated Bcl-xL is virtually absent in young platelets and accumulates in old platelets. Therefor the Asn52Asn66 doubly-deamidated Bcl-xL could be used as a reliable biomarker for determining the age of platelets.