A genome-replication-deficient and transcription/competent negative/strand RNA virus, which can be used for the expression of transgenes and in particular for the area of vaccine development. In particular, a recombinant Sendia virus containing a viral genome with a mutation in the gene P, which leads to loss of capacity for replication without loss of capacity for secondary transcription, and at least one sequence coding for a heterologous gene product. Furthermore, a nucleocapsid of the Sendai virus, a genome of the Sendai virus, a DNA molecule that codes for the genome and/or antigenome of the Sendai virus. One aspect is a pharmaceutical composition containing the Sendai virus, the nucleocapsid, or the viral genome as active substance, and a medical use thereof, especially as a vaccine.