HAL (histidine ammonia-lyase) is a cytosolic enzyme that catalyzes the nonoxidative deamination of histidine to urocanic acid, representing a critical step in histidine catabolism 1. The HAL gene is a single-copy gene spanning approximately 25 kb with 21 exons located on chromosome 12, with transcription regulated by hepatic and epidermis-specific transcription factors including C/EBP, NFIL6, and HNF5 1. HAL deficiency causes histidinemia, an inborn metabolic error characterized by reduced histidase activity and accumulation of histidine and urocanic acid 1. This condition represents the most frequent inborn metabolic error in Japan 1. The molecular characterization of the HAL gene has facilitated investigation of both symptomatic and asymptomatic forms of histidinemia, with identified polymorphisms in exon 16 enabling genetic screening 1. Understanding HAL function and mutations is essential for diagnosing and potentially developing therapeutic strategies for histidinemia, though specific clinical manifestations and long-term management approaches require further investigation beyond the currently available literature.