Pelizaeus–Merzbacher-like disease (PMLD) is a heterogeneous disease with primary hypomyelination of the central nervous system. Only the minority of patients have mutations in the coding region of the GJA12 gene encoding gap junction protein alpha 12, a subunit of intercellular channels highly expressed by oligodendrocytes, the myelin forming cells of the central nervous system. No other gene has been found so far to be mutated in PMLD besides GJA12. We therefore extended the mutational screening in the GJA12 gene, searched for alternative first exons—as described in mice—determined the human 5′-end of the gene, screened therein for mutations and analyzed for copy number variations of the GJA12 gene in 14 patients with PMLD. Unlike in mice we did not find alternative first exons but detected a unique 79 bp first exon in human adolescent brain and spinal cord. No mutation in this non-coding region was found in our cohort. Copy number variation of the GJA12 gene was assessed by real-time PCR TaqMan® gene expression technology, but neither patient showed an aberrant copy number. These data confirm that GJA12 alterations are a rare cause of PMLD—even after extending the screening for copy number variation and for mutations in the non-coding region of GJA12. Full genome scans in informative families and further screenings of candidate genes are feasible approaches to elucidate the genetic background of the majority of patients with PMLD. © 2008 Wiley-Liss, Inc.