LCT and congenital secretory chloride diarrhea 1: While the clinical and biochemical syndrome of CLD had been described as early as the 1960s, the LCT gene was not discovered until 1998 by Jarvela et al. Cases of congenital lactase deficiency have been identified across many ethnicities; however, the majority of mutations identified have been in individuals of Finnish ancestry (Järvelä et al. 1998; Diekmann et al. 2015; Wanes et al. 2019).