176It must be recalled that a breach of the principle of equal treatment, applicable to the law relating to the employment of EU officials, occurs when two categories of person whose factual and legal circumstances disclose no essential difference are treated differently and that difference in treatment is not objectively justified. In adopting applicable rules, especially in the sphere of the EU civil service, the legislature is obliged to observe the general principle of equal treatment (see judgment in Centeno Mediavilla and Others v Commission, cited in paragraph 94 above, EU:C:2008:767, paragraphs 76 and 78 and the case-law cited).