Because DA deficit is a hallmark defect in HPRT deficiency and because we and other laboratories have previously demonstrated that HPRT deficiency is accompanied by dysregulated expression of several of these important transcription factors [13], [14], including Lmx1a and En1&2, we examined the expression of Lmx1a and En1 in basal and differentiated control and knockdown cells (Figure 2). This evidence concerns the gene HPRT1 and hyperinsulinemic hypoglycemia, familial, 4.