Abstract: An antibody that binds a MeCP2E1 isoform of MeCP2 protein, wherein the antibody comprises a region comprising the amino acid sequence of SEQ ID NO:4. A method for detecting and/or monitoring a disease or a disorder caused by an over-expression or an under-expression of a MeCP2E1 isoform of MeCP2 protein, comprising the steps of: (i) obtaining a first sample from a mammalian subject; (ii) contacting the first sample with the anti-MeCP2E1antibody; (iii) removing unbound antibody from the sample; (iv) conducting an immunoassay on the first sample to determine a first value for expression of the MeCP2E1 isoform; (v) comparing the first value to a reference value for expression of the MeCP2E1 isoform in healthy mammalian subjects; wherein a deviation of the first value from the reference value indicates the presence of a disease or a disorder caused by an over-expression or an under-expression of the MeCP2E1 isoform.