HAL and skin cancer: Unexpectedly, although we anticipated that the effect of the rs3819817 HAL eQTL variant on vitamin D level and skin cancer risk would be mediated by the conversion of histidine to trans-urocanate (Fig. 4), the variant had only a weak association with plasma histidine level (beta = −0.014; P = 1.8 × 10−6; Fig. 3c).