The Hippo signaling pathway plays a pivotal role during early mammalian embryonic development, especially in determining the first cell fate decision between the inner cell mass (ICM) and the trophoblast. This pathway operates through a complex cascade involving several key proteins, including LATS kinase, YAP, and the transcription factor Tead4.

In inner blastomeres destined to become the ICM, Hippo signaling is active. LATS kinase phosphorylates the transcriptional coactivator YAP, preventing it from entering the nucleus. The phosphorylated YAP is retained in the cytoplasm and targeted for degradation, thus inhibiting its function. As a consequence, YAP cannot bind Tead4 to activate downstream targets such as the Cdx2 gene, which promotes trophoblast formation.

Therefore, in cells where Hippo signaling is active and YAP is phosphorylated and excluded from the nucleus, Cdx2 expression is suppressed, leading to the formation of the ICM.

The correct statement reflecting this mechanism is:

(2) If LATS kinase phosphorylates the YAP transcriptional coactivator, the phosphorylated form of YAP does not enter the nucleus and gets degraded which leads to ICM formation.

This inhibitory control by the Hippo pathway ensures proper segregation of the first embryonic lineages, crucial for normal development and subsequent differentiation processes.

Answer: (2) If LATS kinase phosphorylates the YAP transcriptional coactivator, the phosphorylated form of YAP does not enter the nucleus and gets degraded which leads to ICM formation.