Amino acid starvation is a pathophysiological condition that results in protein deprivation due to cancer cachexia. Using the method of differential display of reverse transcription PCR (DDRT-PCR), we isolated a cDNA fragment in MCF-7 human breast cancer cells in response to amino acid starvation, which was identical with human CD24 gene. Northern blot results showed that CD24 mRNA in MCF-7 cells was constitutively expressed and significantly upregulated upon amino acid starvation. This stimulation was time-dependent and the maximal response was at 24 h. The expression of the amino acid starvation-induced CD24 mRNA decreased when starved cells were returned to a medium supplemented with amino acids. This repressive response was also time-dependent. Amino acid starvation-induced CD24 mRNA expression in MCF-7 cells was completely blocked by actinomycin D, which suggested that the regulation of CD24 mRNA by amino acid availability occurred at transcriptional level. When amino acid-starved cells were refed with amino acids for 8 h, the expression of CD24 mRNA declined to the basal levels confirming that CD24 mRNA expression could be stimulated by amino acid starvation. Interestingly, CD24 mRNA was poorly detected in MCF-10 cells, a benign human breast epithelial cell line. In conclusion, CD24 mRNA expression in MCF-7 cells was upregulated upon amino acid starvation. This amino acid starvation-induced upregulation of CD24 mRNA occurred at transcriptional level. The regulation of CD24 mRNA in MCF-7 cells by amino acid availability may play an important role in the progression and metastasis of human breast cancer.