Proximal phalanges of digit 1 have different identities in the forelimbs and hindlimbs of the mouse

Mice possess hands and feet similar to humans including two phalanges for digit 1 and three phalanges in digits 2 to 5. The phalanges that make up the digits were named by their positions, however, phalanx identities between digits are unconfirmed by genetic evidence. Here we show that the proximal phalanges of digit 1 of the fore- and hindlimbs have different identities compared to the phalanges of digit 2. We found strong signals from mRNA-Seq results that unite the proximal phalanx of forelimb digit 1 with the proximal phalanx of forelimb digit 2, while the proximal phalanx of hindlimb digit 1 groups the middle phalanx of hindlimb digit 2. The same result was obtained when other clustering methods were used, indicating that our results are robust. Our results demonstrate that there are developmental differences among phalanges and that the proximal phalanx identities of digit 1 differ between the fore- and hindlimbs of the mouse.