Non-coding transcripts originating upstream of the immunoglobulin constant region (I-transcripts) are required to direct activation-induced deaminase to initiate class switching in B cells. Differential regulation of Iε and Iγ1 transcription in response to interleukin-4 (IL-4), hence class switching to IgE and IgG1, is not fully understood. Here we combine novel mouse reporters and single-cell RNA-seq to reveal the heterogeneity in IL-4 induced I-transcription. Transgenic mice were generated with one allele labelled with an eGFP fluorescent reporter gene in the first exon of the gamma-1 non-coding RNA and one allele with a tdTom fluorescent reporter gene in the first exon of the epsilon non-coding RNA. Ex vivo splenic B cells from these mice were stimulated with IL-4, lipopolysaccharide and B cell activating factor. After 24 hours culture, cell were sorted according to their fluorescence and used for single-cell mRNA-sequencing. Two independent experiments were performed (labelled as 'batch 1' and 'batch 2').