An I-CreI variant, wherein one of the I-CreI monomers has at least two substitutions, one in each of the two functional subdomains of the LAGLIDADG core domain situated respectively from positions 26 to 40 and 44 to 77 of I-CreI, said variant being able to cleave a DNA target sequence from a RAG gene. Use of said variant and derived products for the prevention and the treatment of a SCID syndrome associated with a mutation in a RAG gene.