<span class="paragraphSection"><div class="boxTitle">Abstract</div>CRISPR interference (CRISPRi) screens have emerged as powerful tools for dissecting gene function, yet their application to genes with multiple promoters, which comprise over 60% of human genes, remains poorly understood. Here, we demonstrate that CRISPR-dCas9-based screens exhibit widespread promoter specificity, with untargeted promoters often showing compensatory upregulation to maintain gene expression. Leveraging this selective targeting of individual promoters within the same gene, we developed Isoform-Specific single-cell Perturb-Seq to systematically analyse alternative promoter function. Our analysis revealed that alternative promoters in 51.6% of targeted genes drive distinct transcriptional programs. This suggests that promoter selection represents a fundamental mechanism for generating cellular diversity rather than mere transcriptional redundancy. In breast cancer models, this promoter-specific targeting revealed differential effects on drug sensitivity, where distinct estrogen receptor (<span style="font-style: italic;">ESR1</span>) promoters showed opposing influences on tamoxifen response and patient survival. These findings demonstrate the necessity of promoter-level analysis in functional genomics and suggest new strategies for therapeutic intervention through promoter-specific targeting.</span>