['In nonhuman mammals, lactase activity declines during or after weaning. In contrast, about one half of the human species maintains high lactase activity even in adulthood. To clarify this difference, this study examined some parameters for which contrasting observations have been reported in connection with lactase decline.', 'Lactase activity, lactase messenger RNA (mRNA) levels, and in vitro lactase biosynthesis were determined in normal jejunal samples from a large group of white adults, all born in or near Naples.', 'Of 44 individuals, 10 were lactase persistent and 34 were hypolactasic. Biosynthesis of prolactase correlated well with lactase mRNA levels, indicating transcriptional control; it did less so with steady-state lactase activity. Examination of lactase mRNA levels and lactase activity/lactase mRNA ratios revealed a heterogeneous pattern of lactase mRNA level, lactase synthesis, and activity in both lactase persistent and hypolactasic subjects.'] Are lactase persistence versus decline in human adults : multifactorial events involved in down-regulation after weaning?