Analysis of a Hand1 hypomorphic allele reveals a critical threshold for embryonic viability.

Loss-of-function analysis of the basic helix-loop-helix (bHLH) transcription factor Hand1 indicates critical roles in development. In an effort to generate a Hand1 cDNA knock-in reporter mouse, we generated two hypomorphic alleles, which extend embryonic survival to between embryonic day (E) 10.5 and E12.