Horvitz lab (Ruvkun et al., 1989), and Gary went on to mid 1970s through the isolation of a mutation (e912). sequence the lin-14 (a novel nuclear protein) gene in his The remarkable developmental defects of lin-4 (e912) own lab at MGH (Ruvkun and Giusto, 1989). Gary’s lab were first described by Horvitz and Sulston (1980) and discovered that the n355 and n536 gain-of-function mucharacterized in detail by Chalfie et al.(1981). lin-4 (e912) tations are deletions in the 3! untranslated region (UTR) animals look terrible: they grow into long, thin “adults” of the lin-14 mRNA, and that LIN-14 protein level is with a larval skin, and they fail to stop molting at the posttranscriptionally downregulated during worm denormalstageandthusundergoextralarvalstages. Chalvelopment (Wightman et al., 1991). Therefore, if lin-4 fie et al.(1981) showed that e912 hermaphrodites and were involved in the temporal regulation of lin-14, it males are completely missing many of the cell types would probably do so via the lin-14 3! UTR. and morphological structures typical of the wild-type Despite the intriguing correspondence between lin-adults, and instead contain many extra copies of cells

14 and lin-4 mutant phenotypes, we were not really sure ordinarily produced only at an early larval stage. It apthat the cloning and molecular characterization of lin-4 peared that the e912 mutation was causing a failure of would be a worthwhile project, because the lin-4 (e912) temporal developmental switches throughout the animutation was the only known mutant allele of lin-4. If mal, indicating that lin-4 might encode a master regula-lin-4 were a normal worm gene, we knew that knockout tor of developmental timing. alleles should have been more easily recovered in For us, a particularly alluring feature of lin-4 was its screens for egg-laying defective mutants. Perhaps e912 genetic relationship with lin-14. lin-14 was discovered was not a simple loss-of-function mutation in a regulaby Edwin (Chip) Ferguson, a graduate student in Bob tory gene. What if e912 were a rare, arcane genetic Horvitz’s lab. Chip was characterizing genetic pathways rearrangement that disturbed development in a fashion controlling steps in development of the C. elegans herunrelated to normal gene activity? In that case, molecumaphrodite vulva (Ferguson et al., 1987). lin-4 (e912) her- lar analysis of e912 would not teach us anything fundamaphrodites lack even a hint of a vulva (owing to their mental about normal development. On the other hand, failure to generate appropriately specified vulva precur- an optimistic view was that lin-4 might be a very small, sor cells) and hence are unable to lay their eggs (which but otherwise conventional, gene. A gene encoding a consequently hatch inside their mother and consume very short protein might present a very small target for her). While growing cultures of lin-4 (e912) animals for mutagenesis, explaining the scarcity of lin-4 loss-ofgenetic experiments, Chip serendipitously discovered function alleles. We do not recall thinking that lin-4 might a spontaneous revertant that displayed nearly normal encode a small regulatory RNA until much later on, when morphology and egg-laying. Chip determined that the our data finally forced us to consider the possibility. responsible suppressor mutation was in a previously The lin-4 cloning project in Victor’s Harvard lab began unknown gene, lin-14. Later, Victor, as a postdoc work- in the summer of 1988, when a postdoctoral fellow, ing in the Horvitz lab, identified apparent null alleles of Xianjie Yang, conducted genetic mapping experiments with chromosomes polymorphic for RFLPs in the lin-4 region. Xianjie shifted to other pursuits at the …